[release-branch.go1.11] cmd/compile: fix deriving from x+d >= w on overflow in prove pass

In the case of x+d >= w, where d and w are constants, we are
deriving x is within the bound of min=w-d and max=maxInt-d. When
there is an overflow (min >= max), we know only one of x >= min
or x <= max is true, and we derive this by excluding the other.
When excluding x >= min, we did not consider the equal case, so
we could incorrectly derive x <= max when x == min.

Updates #29502.
Fixes #29503.

Change-Id: Ia9f7d814264b1a3ddf78f52e2ce23377450e6e8a
Reviewed-on: https://go-review.googlesource.com/c/156019


Reviewed-by: David Chase <drchase@google.com>
(cherry picked from commit 2e217fa7)
Reviewed-on: https://go-review.googlesource.com/c/163724


Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>