[clang] [FMV][AArch64] Don't optimize backward compatible features in resolver. (PR #90928)

[Sam Elliott via cfe-commits]

lenary wrote:

Is this check even right for MTE? FEAT_MTE uses encodings that are undefined (rather than nop-compatible) in the base architecture, even though those encodings are not doing tag checking until you enable at least FEAT_MTE2 - so I cannot execute e.g. `IRG` on a base armv8.0a architecture.

https://github.com/llvm/llvm-project/pull/90928