Implement a TODO: for any shuffle that can be viewed as a v4[if]32 shuffle,

Implement a TODO: for any shuffle that can be viewed as a v4[if]32 shuffle,
if it can be implemented in 3 or fewer discrete altivec instructions, codegen
it as such.  This implements Regression/CodeGen/PowerPC/vec_perf_shuffle.ll

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@27748 91177308-0d34-0410-b5e6-96231b3b80d8