cmd/compile: use Bool accessor in place of Val.U.(bool)

We have a Bool() accessor for the value in boolean nodes, that we use
elsewhere for n.Val().U.(bool), use it here too. Noticed while reading
the code.

Change-Id: Ie42e014970099a05fe9f02f378af77b63e7e6b13
Reviewed-on: https://go-review.googlesource.com/c/go/+/261360


Trust: Alberto Donizetti <alb.donizetti@gmail.com>
Run-TryBot: Alberto Donizetti <alb.donizetti@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>