[clang] [llvm] [AArch64] Fix feature flags dependecies (PR #90612)

[Ahmed Bougacha via llvm-commits]

ahmedbougacha wrote:

> The overall idea is that system registers ought be available everywhere without the need to explicitly enable them with a command line option.

Interesting, that's kind of what I'm getting at: longer term, are you saying you folks are considering removing the feature predicates on all system registers (i.e., `AArch64SystemOperands.td`), or are you thinking of treating FPMR specifically as a first-class register, the way we do NZCV or FPCR?  I'd be curious to see how the latter goes, since it seems likely to be almost required for any kind of meaningful FP8 codegen...

As for sysregs in general, I think we'd still want the ability to have predicates, in part because some of them can have conflicting encodings, in part to catch obvious mistakes when writing asm;  the specific subtarget features would matter.  But yeah, on most of them, mainly the architected ones, it's probably fine to do away with feature requirements.

https://github.com/llvm/llvm-project/pull/90612