cmd/compile: fix prove pass when upper condition is <= maxint

cmd/compile: fix prove pass when upper condition is <= maxint

When the terminating condition is <= X, we need to make sure that
X+step doesn't overflow.

Fixes #53600

Change-Id: I36e5384d05b4d7168e48db6094200fcae409bfe5
Reviewed-on: https://go-review.googlesource.com/c/go/+/415219
Reviewed-by: Than McIntosh <thanm@google.com>
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>