cmd/compile: more minor cleanup in shortcircuitBlock
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 <cdc248cc4d32e4fbfe04e45228163bcecd48a0b9@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>

cloudflare/go,nawawi/go,cloudflare/go,CAFxX/go,nawawi/go,akutz/go,CAFxX/go,nawawi/go,akutz/go,cloudflare/go,akutz/go,CAFxX/go,cloudflare/go,CAFxX/go,golang/go,CAFxX/go,golang/go,cloudflare/go,golang/go,akutz/go,nawawi/go,akutz/go,CAFxX/go,golang/go,nawawi/go,CAFxX/go,akutz/go,cloudflare/go,golang/go,CAFxX/go,akutz/go,golang/go,golang/go,nawawi/go,golang/go,cloudflare/go,nawawi/go,cloudflare/go,nawawi/go,akutz/go