Added tests for shufflevector lowering to blend instrs.

These tests ensure that a change I will propose in clang works as
expected.

Summary:
Added tests for the generation of blend+immediate instructions from a
shufflevector.
These tests were proposed along with a patch that was dropped. I'm
committing the tests anyway to protect against possible regressions in
codegen.

Reviewers: nadav, bkramer

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D3600