[DAGCombiner] fold vselect-of-constants to shift

[DAGCombiner] fold vselect-of-constants to shift

The diffs suggest that we are missing some more basic
analysis/transforms, but this keeps the vector path in
sync with the scalar (rL374397). This is again a
preliminary step for introducing the reverse transform
in IR as proposed in D63382.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@374555 91177308-0d34-0410-b5e6-96231b3b80d8