[X86] Add DAG combine to turn (trunc (srl (mul ext, ext), 16) into PMULHW/PMULHUW.

Ultimately I want to use this to remove the intrinsics for these instructions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@330520 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed