[DAGCombiner] Combine shuffles of BUILD_VECTOR and SCALAR_TO_VECTOR

This patch attempts to fold the shuffling of 'scalar source' inputs - BUILD_VECTOR and SCALAR_TO_VECTOR nodes - if the shuffle node is the only user. This folds away a lot of unnecessary shuffle nodes, and allows quite a bit of constant folding that was being missed.

Differential Revision: http://reviews.llvm.org/D8516

llvm-svn: 234004