Inliner: enable inlining of methods with conditional throws

[AndyAyersMS]

Remove inlining limitation for methods that return values and have
conditional throws. This limitation was most likely a vestige of an
older inlining implementation that did not break trees at inline
call sites.

Also removed the now-unused observation and the seenConditionalJump
member variable. Merged ifdef blocks in impInit.

Ran full desktop testing, no issues.

Enables a handful of inlines in the various code size suites. For the
most part these slightly increase code size but can often shorten the
non-EH paths in the code.

[AndyAyersMS]

cc @dotnet/jit-contrib

Output from jit-diff:

 bytes of diff: 1381 (0.02 % of base)
    diff is a regression.
Total byte diff includes 0 bytes from reconciling methods
        Base had    0 unique methods,        0 unique bytes
        Diff had    0 unique methods,        0 unique bytes
Top file regressions by size (bytes):
         994 : System.Private.CoreLib.dasm (0.03 % of base)
         330 : System.Net.Sockets.dasm (0.19 % of base)
          57 : System.Runtime.Extensions.dasm (0.07 % of base)
3 total files with size differences.
Top method regessions by size (bytes):
         191 : System.Private.CoreLib.dasm - mincore:GetMessage(long,int):ref
         164 : System.Private.CoreLib.dasm - mincore:TryGetErrorMessage(long,int,ref,byref):bool
         154 : System.Private.CoreLib.dasm - System.Guid:TryParseGuidWithNoStyle(ref,byref):bool
         150 : System.Private.CoreLib.dasm - System.Globalization.DateTimeFormatInfo:InsertAtCurrentHashNode(ref,ref,char,int,int,int,int,int):this
         132 : System.Private.CoreLib.dasm - System.Threading.ManualResetEventSlim:Wait(int,struct):bool:this
13 total methods with size differences.

Example code diff:

;;; Assembly listing for method System.AppDomain:ExecuteAssembly(ref):int:this
;;; Before

G_M56995_IG01:
       sub      rsp, 40
       nop      
G_M56995_IG02:
       xor      r8d, r8d
       mov      dword ptr [rsp+20H], r8d
       xor      r8, r8
       xor      r9, r9
       call     [System.AppDomain:ExecuteAssembly(ref,ref,ref,int):int:this]
       nop      
G_M56995_IG03:
       add      rsp, 40
       ret  

;;; After

G_M56995_IG01:
       push     rsi
       sub      rsp, 32
G_M56995_IG02:
       test     rdx, rdx
       je       SHORT G_M56995_IG05
G_M56995_IG03:
       xor      eax, eax
G_M56995_IG04:
       add      rsp, 32
       pop      rsi
       ret      
G_M56995_IG05:
       call     [CORINFO_HELP_READYTORUN_NEW]
       mov      rsi, rax
       mov      ecx, 425
       call     CORINFO_HELP_STRCNS_CURRENT_MODULE
       mov      rdx, rax
       mov      rcx, rsi
       call     [System.ArgumentNullException:.ctor(ref):this]
       mov      rcx, rsi
       call     CORINFO_HELP_THROW
       int3

[omariom]

So now methods like the following will be inlined?

private int GetLength(string str)
{
   if (str == null)
      throw new ArgumentNullException();

   return str.Length:
}

[AndyAyersMS]

@omariom the conditional throw will no longer block inlining, so a simple method like this should get inlined.

[AndyAyersMS]

@dotnet/jit-contrib anyone up for a review? It is a pretty small change...

[erozenfeld]

LGTM

[AndyAyersMS]

Ignoring the jit diff failure.