Lower vperm(x,y, mask) -> shuffle(x,y,mask) if mask is constant. This allows

Lower vperm(x,y, mask) -> shuffle(x,y,mask) if mask is constant.  This allows
us to compile oh-so-realistic stuff like this:

 vec_vperm(A, B, (vector unsigned char){14});

to:
        vspltb v0, v0, 14

instead of:

        vspltisb v0, 14
        vperm v0, v2, v1, v0

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@27452 91177308-0d34-0410-b5e6-96231b3b80d8