[X86] Add matching for another pattern of PMADDWD.

Summary:
This is the pattern you get from the loop vectorizer for something like this

int16_t A[1024];
int16_t B[1024];
int32_t C[512];

void pmaddwd() {
  for (int i = 0; i != 512; ++i)
    C[i] = (A[2*i]*B[2*i]) + (A[2*i+1]*B[2*i+1]);
}

In this case we will have (add (mul (build_vector), (build_vector)), (mul (build_vector), (build_vector))). This is different than the pattern we currently match which has the build_vectors between an add and a single multiply. I'm not sure what C code would get you that pattern.

Reviewers: RKSimon, spatel, zvi

Reviewed By: zvi

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D49636

llvm-svn: 338097