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

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

I agree its different to what we do for other register types, and I don't intend to touch those - they were written with 'r' in mind from the start - but the usage 'k' doesn't appear to be consistent, and it makes procedurally determining what suffix to use very messy. I did look at converting all uses of 'k' to 'r' instead, but that was a lot more intrusive: renaming all the KMOV instructions to indicate GPR/KMASK not just the suffix and affecting all the avx512 predicated instructions as well.

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