Use PMADDWD to expand reduction in a loop

Summary:
PMADDWD can help improve 8/16 bit integer mutliply-add operation performance for cases like:

for (int i = 0; i < count; i++)
  a += x[i] * y[i];

Reviewers: wmi, davidxl, hfinkel, RKSimon, zvi, mkuper

Reviewed By: mkuper

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D31679

llvm-svn: 299776