Fix incoming arg struct splitting

[BruceForstall]

This only affects Windows/arm64 with varargs.

For incoming structs split between register and stack, handle the splitting
correctly.

Fixes #57606

[ghost]

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Issue Details

---

This only affects Windows/arm64 with varargs.

For incoming structs split between register and stack, handle the splitting
correctly.

Fixes #57606

Author:	BruceForstall
Assignees:	-
Labels:	area-CodeGen-coreclr
Milestone:	-

[BruceForstall]

Note this comment in lvaInitUserArgs:

runtime/src/coreclr/jit/lclvars.cpp

Lines 724 to 741 in cd88b84

	#if defined(TARGET_ARM64)
	if (compFeatureArgSplit())
	{
	// On arm64 Windows we will need to properly handle the case where a >8byte <=16byte
	// struct is split between register r7 and virtual stack slot s[0]
	// We will only do this for calls to vararg methods on Windows Arm64
	//
	// !!This does not affect the normal arm64 calling convention or Unix Arm64!!
	if (this->info.compIsVarArgs && argType == TYP_STRUCT)
	{
	if (varDscInfo->canEnreg(TYP_INT, 1) && // The beginning of the struct can go in a register
	!varDscInfo->canEnreg(TYP_INT, cSlots)) // The end of the struct can't fit in a register
	{
	cSlotsToEnregister = 1; // Force the split
	}
	}
	}

[BruceForstall]

@jakobbotsch @dotnet/jit-contrib PTAL

[BruceForstall]

No diffs