Add a late IR expansion pass for the experimental reduction intrinsics.

Add a late IR expansion pass for the experimental reduction intrinsics.

This pass uses a new target hook to decide whether or not to expand a particular
intrinsic to the shuffevector sequence.

Differential Revision: https://reviews.llvm.org/D32245

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@302631 91177308-0d34-0410-b5e6-96231b3b80d8