Resynchronize isLoadFromStackSlot with LoadRegFromStackSlot (and stores) in PPCInstrInfo

These functions should have the same list of load/store instructions. Now that
all load/store forms have been normalized (to single instructions or pseudos)
they can be resynchronized.

Found by inspection, although hopefully this will improve optimization.  I've
also added some comments.

llvm-svn: 178180