Don't force unused reg arg to the stack.

[CarolEidt]

No description provided.

[CarolEidt]

@dotnet/jit-contrib PTAL
This is one of those "I can't believe I haven't noticed this before" issues. I found it while working on #43870.
When we had an unused incoming register parameter, we would choose not to allocate a register for it, but that meant that it was uselessly stored to the stack - adding both code and frame size.
Instead, we want to "allocate" a register for it, i.e. the one it's in, and because it's a "last use" it will then be freed.
This only impacts "independently promoted" structs; unused scalar args in registers are not considered candidates. (It might be better, in the long run, to "depromote" unused structs, but this is a cheap and straightforward way to handle them for now).
Here are the diffs. No diffs for x86 (no structs passed in regs) or for arm32 (presumably no unused HFAs):

Arch	OS	What	Delta	Methods Improved	Methods Regressed
Arm64	Windows	Crossgen fx+benchmarks	-1448 (-0.00%)	261	0
x64	windows	Crossgen fx+benchmarks	-11519 (-0.03%)	1486	0
x64	Linux	Crossgen fx+benchmarks	-2487 (-0.01%)	256	0

[sandreenko]

LGTM, just a few nits.

[sandreenko]

Nit: would it be correct to write the condition as

 else if ((refType == RefTypeParamDef) && (varDsc->lvRefCntWtd() <= BB_UNITY_WEIGHT) &&
(!currentRefPosition->lastUse || (currentInterval->physReg == REG_STK)))

so it takes 2 lines instead of 3?

[sandreenko]

Nit: It was not clear for me if not was applicable to the word in the brackets: it is *not* a last used (i.e. unused) should be read as it is not unused or it is unused? From the context, it looks like the first but maybe then we can delete double negation?

Suggested change

	// If this is a low ref-count parameter, and either it is *not* a last used (i.e. unused) or it's
	// If this is a low ref-count parameter, and either it is used (def is not the last use) or it's

[CarolEidt]

Thanks for the review @sandreenko!