Modify the code that lowers shuffles to blends from using blendvXX to vblendXX.

Modify the code that lowers shuffles to blends from using blendvXX to vblendXX.
blendv uses a register for the selection while vblend uses an immediate.
On sandybridge they still have the same latency and execute on the same execution ports.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@154396 91177308-0d34-0410-b5e6-96231b3b80d8