Add support for scalarizing/splitting vector bswap.

Summary:
  SLP Vectorization of intrinsics (r203707) has exposed cases where the
  expansion of vector bswap is failing (PR19151).

Reviewers: hfinkel

CC: chandlerc

Differential Revision: http://llvm-reviews.chandlerc.com/D3104

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204163 91177308-0d34-0410-b5e6-96231b3b80d8