optimize: fix some of those changes.

optimize: fix some of those changes.

For loops with predecrements that we eliminated because we don't want to
rely on the loop index going negative if it's zero before the
predecrement - the index is now unsigned, so it'll *never go negative -
we revert to a similar loop, but with the test checking whether the
index is 0 and decrementing it as the first action in the loop body.

(Yeah, it means that, on machines with condition codes, you don't get to
use the condition code setting of the decrement "for free"; we'll let
the compiler and the processor figure out how to do that efficiently.)