cmd/compile: more minor cleanup in shortcircuitBlock

Continue to simplify, rename for clarity,
improve docs, and reduce variable scope.

This is in preparation for this function becoming
more complicated.

Passes toolstash-check.

Updates #37608

Change-Id: I630a4e07c92297c46d18aea69ec29852d6371ff0
Reviewed-on: https://go-review.googlesource.com/c/go/+/222919


Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>