[Wasm RyuJit] Crossgen fixes

[AndyAyersMS]

Fix some issues that I noticed while trying to crossgen debug/release System.Private.Corelib

• need to be more careful fetching layout when rewriting local stores
• handle degenerate BBJ_COND during wasm flow graph reordering
• pinvoke prolog can likely be empty
• fix VN assert for the extra PE arg we pass at end of arglist
• make sure Wasm special call args have names in dumps
• handle struct arg passed as field during reg alloc
• fix logic for marking this as address-exposed

[AndyAyersMS]

@kg PTAL
FYI @dotnet/jit-contrib

@SingleAccretion take a look at the changes in regalloc.. there may be more fallout from passing structs as their fields.

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[Copilot]

Pull request overview

This PR fixes several issues discovered while attempting to crossgen System.Private.CoreLib for the WASM RyuJit backend. These are targeted bug fixes that address correctness issues in WASM-specific code paths and improve code robustness.

Changes:

• Fixed value numbering assert to account for the WasmPortableEntryPoint argument that is not included in VNFuncArity
• Corrected struct field parent lookup during register allocation for ABI information
• Added proper handling for degenerate conditional branches during flow graph reordering
• Fixed this pointer address-exposed detection to use compThisArg instead of hardcoded varNum==0
• Added Wasm special argument names for debugging output
• Added GT_PINVOKE_PROLOG case handling for WASM codegen (currently empty but prevents NYI assertion)
• Fixed layout fetching logic to only call GetLayout for TYP_STRUCT types

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/coreclr/jit/valuenum.cpp	Fixed assertion to allow WasmPortableEntryPoint arg at end of arglist; removed incorrect comment about VNFuncArity
src/coreclr/jit/regallocwasm.cpp	Fixed layout fetch to check for TYP_STRUCT first; fixed struct field parent lookup for ABI info
src/coreclr/jit/gentree.cpp	Added debug names for WasmShadowStackPointer and WasmPortableEntryPoint arguments
src/coreclr/jit/fgwasm.cpp	Added handling for degenerate BBJ_COND blocks (where true and false targets are identical)
src/coreclr/jit/fgbasic.cpp	Fixed this pointer detection to use info.compThisArg instead of hardcoded 0
src/coreclr/jit/codegenwasm.cpp	Added GT_PINVOKE_PROLOG case with TODO comment for potential future work

[Copilot]

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated no new comments.