[PowerPC] Try to simplify a Swap if it feeds a Splat

If we have the situation where a Swap feeds a Splat we can sometimes change the
  index on the Splat and then remove the Swap instruction.

Fixed the test case that was failing and recommit after pulling the original
  commit.

  Original revision is here: https://reviews.llvm.org/D39009

llvm-svn: 316478