cmd/compile: in prove, detect loops with negative increments
To be effective, this also requires being able to relax constraints
on min/max bound inclusiveness; they are now exposed through a flags,
and prove has been updated to handle it correctly.
Change-Id: I3490e54461b7b9de8bc4ae40d3b5e2fa2d9f0556
Reviewed-on: https://go-review.googlesource.com/104041
Run-TryBot: Giovanni Bajo <rasky@develer.com>
TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: David Chase <drchase@google.com>