Teach the DAGCombiner how to fold 'vselect' dag nodes according
to the following two rules:
  1) fold (vselect (build_vector AllOnes), A, B) -> A
  2) fold (vselect (build_vector AllZeros), A, B) -> B



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@198777 91177308-0d34-0410-b5e6-96231b3b80d8