Revert "Fix signed-unsigned semantics in reduce_32"

This reverts commit aaf59b03.

This commit regressed the scaling-test unit test, by apparently allowing
the compiler to emit fused multiply-add instructions in cases they
wouldn't have been allowed before. While using gcc's -ffp-contract=...
flag avoids the issue on amd64, it does not on at least aarch64 and
ppc64.

This is unfortunate, because the commit being reverted resolved
pixman/pixman#43 so we will
reintroduce this failure, but after more than a year without a fix for
the unit test, I think it's time to bite the bullet.

Fixes: pixman/pixman#49