isShuffleMaskLegal, used to determine if it makes sense to turn a
"vector clear" (e.g. pand V, <0, -1, 0, -1> to a shuffle of the vector and
a zero vector.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@27873 91177308-0d34-0410-b5e6-96231b3b80d8