Arm64/Sve: Validation for movprfx in emitter

[kunalspathak]

As noted in #100743 (comment) . Everytime we emit SVE instruction, we should check the last emitted instruction and if it is movprfx, then validate the things required for movprfx placement.

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[a74nh]

priority:2 for RC1 snap : This is a functional correctness issue - if the rules are violated then the use is UNPREDICTABLE.

[mikabl-arm]

Everytime we emit SVE instruction, we should check the last emitted instruction ...

Shouldn't this be done for any instruction and not just SVE instructions?

[a74nh]

Thoughts:

I'll be adding a new function, something like emitInsSanityCheckWithPrevious()

If this check fails what do we do? Is this just an assert and exit?

We need the previous instruction. Calling emitNewInstrCns() (and friends) will set emitLastIns to this new instruction.

One way is to add emitPrevIns tracking (I think I remember us trying to add that with the peephole optimisation). Advantage now is we can call the new checks from emitInsSanityCheck().

If we don't have tracking, then we need to do the checking before creating the new instruction, manually passing in the id, reg1, reg2 etc etc (just like how ReplaceLdrStrWithPairInstr() works). Now we need to call our new check function before calling emitNewInstrCns() and this needs to be done in every Arm64 emit function.