add an initial cut at preinc loads for ppc32. This is broken for ppc64

add an initial cut at preinc loads for ppc32.  This is broken for ppc64
(because the 64-bit reg target versions aren't implemented yet), doesn't
support r+r addr modes, and doesn't handle stores, but it works otherwise. :)

This is disabled unless -enable-ppc-preinc is passed to llc for now.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@31621 91177308-0d34-0410-b5e6-96231b3b80d8