Fixed a bug in lowering shuffle_vectors to insertps

Summary:
We were being too strict and not accounting for undefs.
Added a test case and fixed another one where we improved codegen.

Reviewers: grosbach, nadav, delena

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D4039

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210361 91177308-0d34-0410-b5e6-96231b3b80d8