cmd/compile: checkwidthj all expressions in walk

cmd/compile: checkwidthj all expressions in walk

Instead of playing whack-a-mole finding all
the non-dowidth'd expressions that can sneak
out of the frontend and then deciding on
just the right place to handle them,
use a big hammer.

Fixes #20152

Change-Id: Id452d9e8c4e9585216bd8bf0e0004c85aba4f9f7
Reviewed-on: https://go-review.googlesource.com/42021
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>