Simplify a vpermil* with constant mask.

With a constant mask a vpermil* is just a shufflevector. This patch implements
that simplification. This allows us to produce denser code. It should also
allow more folding down the line.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206801 91177308-0d34-0410-b5e6-96231b3b80d8