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

[Phoebe Wang via llvm-commits]

phoebewang wrote:

> @phoebewang sorry for the bikeshedding but would `bcast32/64` be better do you think? another alternative would be for us to keep to the `{1to8}` pattern that we use in other places, but that'd be a bit more work.

SDM uses `m128` or `m32bcst`. Would it better if we match with this style?

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