[PATCH] [AArch64] Add v8.1a RDMA extension

[Tim Northover]

> after deep research I've come up with usage of v1i8, v1i16, v1i32 as valid contents of FPR8, FPR12, FPR32.

The original AArch64 backend went down that path, and I think it was a
mistake. It ended up adding lots of complexity for not much gain.

I think the proposed plan for actually using these scalar instructions
is by enhancing the AArch64AdvSIMDScalarPass to optimise these cases
when they occur.

Cheers.

Tim.