[llvm] [AArch64] Set predicates for FP/SIMD InstAliases (PR #79033)

[David Green via llvm-commits]

davemgreen wrote:

> I think that we don't currently support `-mgeneral-regs-only` in the same way that GCC does, so that shouldn't prevent us from handling things consistently. The instructions affected by this change are a random mix, I wouldn't expect this bug to be "useful" to anyone currently using `-mgeneral-regs-only` with clang.

Yeah it's about whether it makes using a feature that people are already using even worse. If they are already relying on it then there is a limit to how much we can just say "it is not supported".

I don't think these aliases should be too much of a problem though, and we have the workaround you mentioned in https://github.com/llvm/llvm-project/pull/77817#issuecomment-1895458298. 

> As for how we could implement `-mgeneral-regs-only`, I think we'd need a way to have different Subtargets for the code-generation and assembler. I don't know exactly how that would work, but it would probably be a big change.

This might be useful for the isNeonAvailable predicates too, which were apparently to allow neon inline assembly in streaming sve functions.

https://github.com/llvm/llvm-project/pull/79033