Use lwzu, stwu to tighten more loops.

Use lwzu, stwu to tighten more loops.

Because lwzu or stwu moves the pointer, I can remove an addi
instruction from the loop, so the loop is slightly faster.

I wrote a benchmark in Modula-2 that exercises some of these loops.  I
measured its time on my old PowerPC Mac.  Its user time decreases from
8.401s to 8.217s with the tighter loops.