cmd/compile: rework induction variable detector

cmd/compile: rework induction variable detector

Induction variable detection is still not quite right. I've added
another failing test.

Redo the overflow/underflow detector so it is more obviously correct.

Update #53600
Fixes #53653
Fixes #53663

Change-Id: Id95228e282fdbf6bd80b26e1c41d62e935ba08ff
Reviewed-on: https://go-review.googlesource.com/c/go/+/415874
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: David Chase <drchase@google.com>