X86: Add pattern matching for PMADDWD

In addition to the existing match as part of a loop-reduction, add a
straightforward pattern match for DAG-contained patterns.

Reviewers: RKSimon, craig.topper

Subscribers: llvm-commits

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D41811

llvm-svn: 322446