PowerPC supports pre-increment floating-point load/store instructions, both r+r
and r+i, and we had patterns for them, but they were not marked as legal. Mark
them as legal (and add a test case).
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@228327 91177308-0d34-0410-b5e6-96231b3b80d8
