cmd/compile: tighten bounds for induction variables in strided loops

cmd/compile: tighten bounds for induction variables in strided loops

for i := 0; i < 9; i += 3

Currently we compute bounds of [0,8]. Really we know that it is [0,6].

CL 415874 computed the better bound as part of overflow detection.
This CL just incorporates that better info to the prove pass.

R=go1.20

Change-Id: Ife82cc415321f6652c2b5d132a40ec23e3385766
Reviewed-on: https://go-review.googlesource.com/c/go/+/415937
Reviewed-by: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Keith Randall <khr@golang.org>