JIT: Disallow parameters passed in float regs from being enregistered in int regs

[jakobbotsch]

See #99286 (comment).

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Issue Details

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Just verifying that we never see float reg -> int reg homing on all our targets...

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

[jakobbotsch]

/azp run runtime-coreclr jitstress, runtime-coreclr libraries-jitstress, runtime-coreclr jitstressregs, runtime-coreclr libraries-jitstressregs

[jakobbotsch]

/azp run runtime-coreclr jitstress, runtime-coreclr libraries-jitstress, runtime-coreclr jitstressregs, runtime-coreclr libraries-jitstressregs

[azure-pipelines]

Azure Pipelines successfully started running 4 pipeline(s).

[jakobbotsch]

cc @dotnet/jit-contrib PTAL @BruceForstall

Diffs.

This change is a follow-up to #96439. The logic in that PR is only correct under the assumption that parameters passed in float regs are never homed in integer regs. However, we can enregister structs of fitting size in integer registers if they have only whole uses, so that could still happen for a struct passed in float registers. This is a very rare case because such enregistration only kicks in when the struct parameter also was not promoted.

The right long term fix here is to generalize genFnPrologCalleeRegArgs to handle float and integer register parameters simultaneously. However, genFnPrologCalleeRegArgs is a pretty gnarly function, and I think backporting such a change to it is too risky, so this PR along with #96439 have the changes I am backporting in #98749.