[Wasm RyuJIT] Partially implement genStructReturn

[kg]

Addresses a top assert from #125756
Scenarios other than fieldlist are not implemented because I don't have a repro for them yet.

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[Copilot]

Pull request overview

Adds an initial WebAssembly-specific implementation of CodeGen::genStructReturn to start handling struct returns in the Wasm RyuJIT backend, as part of addressing top asserts seen during wasm crossgen.

Changes:

• Implement partial genStructReturn handling for GT_RETURN where the return value is a FIELD_LIST.
• Consume field list uses to keep liveness consistent, then attempt to return.

[Copilot]

actualOp1, retTypeDesc, and regCount are not used outside of assert. In non-DEBUG builds assert compiles out, leaving these locals unused (and potentially failing the build under warnings-as-errors). Either remove them for now or use (void) casts / use regCount in the logic (e.g., compare against the FIELD_LIST use count).

Suggested change

	(void)actualOp1;
	(void)retTypeDesc;
	(void)regCount;

[kg]

Can confirm that this fixes:

--singlemethodtypename "System.RuntimeTypeHandle" --singlemethodname "FromIntPtr" --singlemethodindex 1
--singlemethodtypename "System.RuntimeTypeHandle" --singlemethodname "GetModuleHandle" --singlemethodindex 1
--singlemethodtypename "System.BitConverter" --singlemethodname "ToBFloat16" --singlemethodindex 1
--singlemethodtypename "System.DateOnly" --singlemethodname "get_MinValue" --singlemethodindex 1

[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated no new comments.

[SingleAccretion]

We should not be emitting the return here, it will be part of the epilog.

I think this method can simply be: foreach (field in fieldList) genConsumeReg(field).

Since we don't have multi-regs, I think the NYI below can be removed and we can assert it is always a field list.