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

[Tim Northover]

  Ah, that was the bit I was missing. I swear I looked at the getConstant implementation in case it did that, but obviously not hard enough.

  That makes it rather easier to think about. I think it looks good.

  Cheers.

  Tim.

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