[llvm] [AMDGPU] Add MovB64 subtarget feature (PR #196340)

[Shilei Tian via llvm-commits]

https://github.com/shiltian approved this pull request.

LGTM. If you really want to avoid changing `hasMovB64` everywhere, you can just call `HasMovB64Inst` in it, but that might not be a good idea.

https://github.com/llvm/llvm-project/pull/196340