[AArch64] Add SubtargetFeatures for v8.7-A options

[pratlucas]

Summary

This adds subtarget features for v8.7-A's limited-tlb-maintenance nXS
instruction qualifiers, for the WFET/WFIT instructions and for the
HCRX_EL2 system register, allowing each to be conditional on their own
SubtargetFeature instead of being conditional on the base "v8.7a" feature.

Based on patches written by Simon Tatham.

[ostannard]

llvm/lib/Target/AArch64/AArch64.td

406

Why is this FeatureHCX, and not FeatureHCRX for consistency with the register name?

llvm/lib/Target/AArch64/AsmParser/AArch64AsmParser.cpp

2928

Why does this need a special case? Would it not be better to add these extensions to ExtensionMap above?

3019

Could we represent these operand variants in the tablegen, to avoid needing to special-case them here. We could make the TLBI tablegen class a multiclass which expands to the regular and nXS variants, with the appropriate feature requirements set.

[simon_tatham]

llvm/lib/Target/AArch64/AArch64.td

406

"HCX" is the name of the field in ID_AA64MMFR_1 that indicates the presence of that register.

[ostannard]

llvm/lib/Target/AArch64/AArch64.td

406

Ok, that's fine then.

[pratlucas]

As suggested, the contents of this revision were squashed into D91772.