[PATCH] Make DAGCombiner work on vector bitshifts with constant splat vectors.

[Tim Northover]

  Well, the name isn't ideal. Most other "getXYZ" functions create something new. I'd probably go for "isConstantSplat" and document that it returns nullptr if not.

  It should probably get some Doxygen comments too, since that's a public interface.

http://llvm-reviews.chandlerc.com/D2997