[clang] [clang-tools-extra] [compiler-rt] [llvm] [X86] AMD Zen 6 Initial enablement (PR #179150)

[Simon Pilgrim via cfe-commits]

https://github.com/RKSimon requested changes to this pull request.

Mainly minor tidyup comments - it'd be much better if the BMM work was in a separate patch to the znver6 work - some BMM ref docs would be good as all I have to go off is the gcc patches. We have enough ISD::BITREVERSE functionality now that I don't see any need for the BITREV builtins/intrinsics.

https://github.com/llvm/llvm-project/pull/179150