[PowerPC] Compute the MMO offset for an unaligned load with signed arithmetic

If you compute the MMO offset using unsigned arithmetic, you end up with a
large positive offset instead of a small negative one. In theory, this could
cause bad instruction-scheduling decisions later.

I noticed this by inspection from the debug output, and using that for the
regression test is the best I can do right now.

llvm-svn: 246805