cmd/compile: clarify Node.NonNil and Node.Bounded

cmd/compile: clarify Node.NonNil and Node.Bounded

Node.NonNil and Node.Bounded were a bit muddled. This led to #38496.
This change clarifies and documents them.

It also corrects one misuse.
However, since ssa conversion doesn't make full use of the bounded hint,
this correction doesn't change any generated code.
The next change will fix that.

Passes toolstash-check.

Change-Id: I2bcd487a0a4aef5d7f6090e653974fce0dce3b8e
Reviewed-on: https://go-review.googlesource.com/c/go/+/228787
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>