[llvm] [X86] Consistently use 'k' for predicate mask registers in instruction names (PR #108780)

[Phoebe Wang via llvm-commits]

phoebewang wrote:

I'm a little concerned if we were intended to use `k` only when `r` for different register type, i.e., masked instructions. We don't distinguish GPR with *MM registers. So would be for K register only instructions?

https://github.com/llvm/llvm-project/pull/108780