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.

[Copilot]

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.

[AndyAyersMS]

Possible regressions in BigInteger: dotnet/perf-autofiling-issues#52301