[X86] Fold (movmsk (setne (and X, (1 << C)), 0)) -> (movmsk (X << C)) for vXi8 vectors.

We don't have a vXi8 shift left so we need to bitcast to a vXi16 vector to perform the shift. If we let lowering legalize the vXi8 shift we get an extra and that we don't need and fail to remove.

llvm-svn: 342795