Patched clang to emit x86 blends as shufflevectors.

Summary:
Most of the clang header patch by Simon Pilgrim @ SCEE.
Also fixed (or added) clang tests for these intrinsics.

LLVM tests to make sure we get the blend instruction out of these
shufflevectors are at http://reviews.llvm.org/D3600

Reviewers: eli.friedman, craig.topper

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D3601