[DAGCombine] Produce better code for constant splats

[DAGCombine] Produce better code for constant splats

This solves PR22276.
Splats of constants would sometimes produce redundant shuffles, sometimes ridiculously so (see the PR for details). Fold these shuffles into BUILD_VECTORs early on instead.

Differential Revision: http://reviews.llvm.org/D7093

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@226811 91177308-0d34-0410-b5e6-96231b3b80d8