cmd/compile: in prove, infer unsigned relations while branching

When a branch is followed, we apply the relation as described
in the domain relation table. In case the relation is in the
positive domain, we can also infer an unsigned relation if,
by that point, we know that both operands are non-negative.

Fixes #20393

Change-Id: Ieaf0c81558b36d96616abae3eb834c788dd278d5
Reviewed-on: https://go-review.googlesource.com/100278


Run-TryBot: Giovanni Bajo <rasky@develer.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Giovanni Bajo <rasky@develer.com>
Reviewed-by: David Chase <drchase@google.com>