Adds code to PPC ISEL lowering to recognize byte inserts from vector_shuffles, and...

Adds code to PPC ISEL lowering to recognize byte inserts from vector_shuffles, and use P9 shift and vector insert byte instructions instead of vperm. Extends tests from vector insert half-word.

Differential Revision: https://reviews.llvm.org/D34497

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@317503 91177308-0d34-0410-b5e6-96231b3b80d8