cmd/compile: fix control flow for unsigned divisions proof relations

cmd/compile: fix control flow for unsigned divisions proof relations

The continue used to make sense since I first wrote this patch with a loop,
for testing the commutativity of the add.

This was refactored to just try both but I forgot to fix the continue.

Change-Id: I91466a052d5d8ee7193084a71faf69bd27e36d2a
Reviewed-on: https://go-review.googlesource.com/c/go/+/721204
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Mark Freeman <markfreeman@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Jorropo <jorropo.pgm@gmail.com>