Implement four-wide shuffle with 2 shufps if no more than two elements come

from each vector. e.g.
        shuffle(G1, G2, 7, 1, 5, 2)
==>
        movaps _G2, %xmm0
        shufps $151, _G1, %xmm0
        shufps $216, %xmm0, %xmm0

llvm-svn: 28011