Clang and AArch64 backend patches to support arm v8 Neon

[Tim Northover]

> Tim (and anyone else interested), can you do the review of the rebased
> patches and merge them?

Oh, and Jim pointed out that RegisterOperand was probably a better way
to handle the NEON registers back in January. I don't think that
necessarily needs to change before commit though, the patches are big
enough as it is.

Tim.