[DAGCombiner] Combine shuffles of BUILD_VECTOR and SCALAR_TO_VECTOR

[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@234004 91177308-0d34-0410-b5e6-96231b3b80d8