[X86] Add a DAG combine to simplify PMULDQ/PMULUDQ nodes

These nodes only use the lower 32 bits of their inputs so we can use SimplifyDemandedBits to simplify them.

Differential Revision: https://reviews.llvm.org/D44375

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@328405 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed