[X86][SSE] Add SimplifyDemandedBitsForTargetNode PMULDQ/PMULUDQ handling

Add X86 SimplifyDemandedBitsForTargetNode and use it to simplify PMULDQ/PMULUDQ target nodes.

This enables us to repeatedly simplify the node's arguments after the previous approach had to be reverted due to PR39398.

Differential Revision: https://reviews.llvm.org/D53643

llvm-svn: 345182