JIT: consider optimizing box followed by GetType

[AndyAyersMS]

Might not be a perf win, but it shows up in exception paths so could help reduce overall code size and jit time.

For instance, inspired by similar code inSystem.ValueTuple:

struct B
{
    public B(double d) { x = d; y = 4.4; }
    double x;
    double y;
    public bool CompareTo(object other)
    {
       if (!(other is B)) throw new Exception("Bad type, expected" + this.GetType().ToString());
       var otherB = (B) other;
       return x == otherB.x && y == otherB.y;
    }
}

the codegen for CompareTo is:

; Assembly listing for method B:CompareTo(ref):bool:this
; Emitting BLENDED_CODE for X64 CPU with AVX
; optimized code
; rsp based frame
; partially interruptible
; Final local variable assignments
;
;  V00 this         [V00,T03] (  6,  3.50)   byref  ->  rdi         this
;  V01 arg1         [V01,T00] (  6,  5.25)     ref  ->  rsi         class-hnd
;* V02 loc0         [V02    ] (  0,  0   )  struct (16) zero-ref
;  V03 tmp0         [V03,T01] (  6,  7.24)     ref  ->  rdx         class-hnd
;  V04 tmp1         [V04,T06] (  4,  0   )     ref  ->  rsi
;  V05 tmp2         [V05,T07] (  3,  0   )     ref  ->  rdi         class-hnd exact
;  V06 tmp3         [V06,T04] (  2,  2   )  double  ->  mm0         V02.x(offs=0x00) P-INDEP
;  V07 tmp4         [V07,T05] (  2,  1.50)  double  ->  mm1         V02.y(offs=0x08) P-INDEP
;  V08 tmp5         [V08,T09] (  2,  0   )     ref  ->  rbx
;  V09 tmp6         [V09,T10] (  2,  0   )     ref  ->  rdx
;  V10 tmp7         [V10,T11] (  2,  0   )     ref  ->  rdx
;  V11 tmp8         [V11,T02] (  3,  6   )   byref  ->  rax
;  V12 OutArgs      [V12    ] (  1,  1   )  lclBlk (32) [rsp+0x00]
;  V13 rat0         [V13,T08] (  3,  0   )     ref  ->  rax
;
; Lcl frame size = 32

G_M6708_IG01:
       57                   push     rdi
       56                   push     rsi
       53                   push     rbx
       4883EC20             sub      rsp, 32
       C5F877               vzeroupper
       488BF9               mov      rdi, rcx
       488BF2               mov      rsi, rdx

G_M6708_IG02:
       488BD6               mov      rdx, rsi
       4885D2               test     rdx, rdx
       7411                 je       SHORT G_M6708_IG03
       48B9105A637FF87F0000 mov      rcx, 0x7FF87F635A10
       48390A               cmp      qword ptr [rdx], rcx
       7402                 je       SHORT G_M6708_IG03
       33D2                 xor      rdx, rdx

G_M6708_IG03:
       4885D2               test     rdx, rdx
       7466                 je       SHORT G_M6708_IG10

G_M6708_IG04:
       48BA105A637FF87F0000 mov      rdx, 0x7FF87F635A10
       483916               cmp      qword ptr [rsi], rdx
       7412                 je       SHORT G_M6708_IG05
       488BD6               mov      rdx, rsi
       48B9105A637FF87F0000 mov      rcx, 0x7FF87F635A10
       E8118C385F           call     CORINFO_HELP_UNBOX

G_M6708_IG05:
       488D4608             lea      rax, bword ptr [rsi+8]
       C4E17B1000           vmovsd   xmm0, qword ptr [rax]
       C4E17B104808         vmovsd   xmm1, qword ptr [rax+8]   ;; seems odd to load this.y here
       C4E17B1017           vmovsd   xmm2, qword ptr [rdi]
       C4E1792ED0           vucomisd xmm2, xmm0
       7A20                 jpe      SHORT G_M6708_IG08
       751E                 jne      SHORT G_M6708_IG08
       C4E17B104708         vmovsd   xmm0, qword ptr [rdi+8]   ;; would expect it to be loaded here
       C4E1792EC1           vucomisd xmm0, xmm1
       0F9BC0               setpo    al
       7A03                 jpe      SHORT G_M6708_IG06
       0F94C0               sete     al

G_M6708_IG06:
       0FB6C0               movzx    rax, al

G_M6708_IG07:
       4883C420             add      rsp, 32
       5B                   pop      rbx
       5E                   pop      rsi
       5F                   pop      rdi
       C3                   ret

G_M6708_IG08:
       33C0                 xor      eax, eax

G_M6708_IG09:
       4883C420             add      rsp, 32
       5B                   pop      rbx
       5E                   pop      rsi
       5F                   pop      rdi
       C3                   ret

G_M6708_IG10:
       48B9105A637FF87F0000 mov      rcx, 0x7FF87F635A10
       E84D6E855F           call     CORINFO_HELP_NEWSFAST   ;; allocate new B on heap
       488BF0               mov      rsi, rax   ;; copy contents to new B
       488D4E08             lea      rcx, bword ptr [rsi+8]
       C4E17A6F07           vmovdqu  xmm0, qword ptr [rdi]
       C4E17A7F01           vmovdqu  qword ptr [rcx], xmm0
       48B9904E59DEF87F0000 mov      rcx, 0x7FF8DE594E90
       E82D6E855F           call     CORINFO_HELP_NEWSFAST
       488BF8               mov      rdi, rax
       B901000000           mov      ecx, 1
       48BA704E637FF87F0000 mov      rdx, 0x7FF87F634E70
       E8862C395F           call     CORINFO_HELP_STRCNS
       488BD8               mov      rbx, rax
       488BCE               mov      rcx, rsi     ;; call GetType on new B on heap, ignore contents
       E8AB5AAC5F           call     System.Object:GetType():ref:this
       488BC8               mov      rcx, rax
       488B00               mov      rax, qword ptr [rax]
       488B4048             mov      rax, qword ptr [rax+72]
       FF10                 call     gword ptr [rax]System.Object:ToString():ref:this
       488BD0               mov      rdx, rax
       488BCB               mov      rcx, rbx
       E8C417A35E           call     System.String:Concat(ref,ref):ref
       488BD0               mov      rdx, rax
       488BCF               mov      rcx, rdi
       E8E9D1A55E           call     System.Exception:.ctor(ref):this
       488BCF               mov      rcx, rdi
       E8B121395F           call     CORINFO_HELP_THROW
       CC                   int3

; Total bytes of code 272, prolog size 10 for method B:CompareTo(ref):bool:this

Also a bit puzzling why we load both x and y from this before doing any compare, seems like we should defer loading y. I suppose that's just how promotion works; all the fields get loaded even if some of the downstream uses are conditional.