[llvm] [X86] Generate `vpmuludq` instead of `vpmullq` (PR #121456)

[Phoebe Wang via llvm-commits]

phoebewang wrote:

> because `SimplifyDemandedVectorElts` simplifies the arguments and we fail the combine to `PMULDQ`.

Is it possible to combine the new patten instead of disble `SimplifyDemandedVectorElts`?

https://github.com/llvm/llvm-project/pull/121456