[llvm] [X86] VPTERNLOG comments - use "mem" just for full width loads and "bst32" / "bst64" (PR #143721)

[Simon Pilgrim via llvm-commits]

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

@egorshamshura please can you tweak this to print m32bcst/m64bcst instead? Sorry about the change but this patch appeared very quickly after the issue was first raised.

https://github.com/llvm/llvm-project/pull/143721