[clang] [CLANG][AArch64]Add Neon vectors for fpm8_t (PR #99865)

[Paul Walker via cfe-commits]

paulwalker-arm wrote:

Is it possible to use TargetExtType for the scalar type given this is a target specific type.  I fully expect LLVM not to support vector's of TargetExtType but I wonder if that can be relaxed given our only use case is to pass them to intrinsics. For anything more exotic we can add intrinsics to cast them to i8 vectors.  Alternatively, we could also use TargetExtType for all fp8 scalar and vector types.

https://github.com/llvm/llvm-project/pull/99865