Ask legalize to promote all vector shuffles to be v16i8 instead of having to

Ask legalize to promote all vector shuffles to be v16i8 instead of having to
handle all 4 PPC vector types.   This simplifies the matching code and allows
us to eliminate a bunch of patterns.  This also adds cases we were missing,
such as CodeGen/PowerPC/vec_splat.ll:splat_h.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@27400 91177308-0d34-0410-b5e6-96231b3b80d8