cmd/compile: fix misassumption about n.Left.Bounded()

cmd/compile: fix misassumption about n.Left.Bounded()

n.Bounded() is overloaded for multiple meanings based on n.Op. We
can't safely use n.Left.Bounded() without checking n.Left.Op.

Change-Id: I71fe4faa24798dfe3a5705fa3419a35ef93b0ce2
Reviewed-on: https://go-review.googlesource.com/c/go/+/228677
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>