JIT: Ensure fgTryMorphStructArg morphs created trees

[jakobbotsch]

In particular we could create LCL_FLD or LCL_VAR nodes for address exposed locals without marking them with GTF_GLOB_REF. This would result in not creating some necessary copies.

Fix #113488

Diffs expected -- regressions from more copies introduced. Some improvements from the morphing when it does good things, and from not creating field lists for some DNER cases.

Pull Request Overview

This PR adds a regression test targeting the JIT behavior to ensure that fgTryMorphStructArg properly morphs created trees, preventing missing copies for address-exposed locals.

• Added a new test file to validate the behavior of fgTryMorphStructArg in both Debug and Release builds.
• The test exercise involves struct field manipulation and vector creation to trigger the intended morphing in the JIT.

[jakobbotsch]

cc @dotnet/jit-contrib PTAL @AndyAyersMS

Diffs. Quite substantial diffs from this change. Overwhelmingly these diffs come from tests, however.
Many of the regressions come from introducing zero/sign-extensions on promoted fields passed as arguments. In many cases these don't need to be normalized in the managed calling convention, and we could consider optimizing that, but I don't expect it to be very costly either.

[AndyAyersMS]

Is this a regression? Wondering if we need to keep a close eye on SetMorphed or something.

[jakobbotsch]

Is this a regression? Wondering if we need to keep a close eye on SetMorphed or something.

Yes, this is a regression from #112612, although from a preexisting bug in fgMorphMultiregStructArgs. We have this IR:

fgMorphTree BB01, STMT00006 (before)
               [000018] --CXG------                         ▌  CALL      void   Program:M4(S0,System.Runtime.Intrinsics.Vector256`1[ushort])
               [000011] ----------- arg0                    ├──▌  LCL_VAR   struct<S0, 4>(AX)(P) V00 loc0         
                                                            ├──▌    int    field V00.F0 (fldOffset=0x0) -> V04 tmp3         
               [000017] --CXG------ arg1                    └──▌  HWINTRINSIC simd32 ushort Create
               [000013] --CXG------                            └──▌  CALL      int    S0:M7():ushort:this
               [000012] ----------- this                          └──▌  LCL_ADDR  byref  V00 loc0         [+0]
                                                                     ▌    int    field V00.F0 (fldOffset=0x0) -> V04 tmp3         

When morphing args we first morph [000011] which properly sets GTF_GLOB_REF. Before #112612 we then had a separate path for single-reg promoted arguments that changes [000011] into LCL_VAR V04 via SetOper + SetLclNum. That preserves GTF_GLOB_REF.

After #112612 we instead call into fgTryMorphStructArg (previously called fgMorphMultiregStructArgs). That one uses fgMorphLclArgToFieldList(node)->SoleFieldOrThis() instead, which does not preserve GTF_GLOB_REF.