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

[Jon Roelofs via cfe-commits]

jroelofs wrote:

I am mildly opposed to this on principle: we should be able to optimize for any feature present in the supplied `-mcpu=`, and accommodating features like this that have a "graceful" fallback sets a bad precedent. If someone wants to run code on a machine, they should set mcpu/march/whatever to the minimal spec and use FMV to opt-in to additional features. This patch bends that rule, and makes FMV an opt-out mechanism, but only for these specific features.

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