cmd/compile: rename a local variable in shortcircuitBlock

v is pretty generic. Subsequent changes will make this function
more complicated, so rename it now, independently, for easier review.

v is the control value for the block (or its underlying phi);
call it ctl.

Passes toolstash-check.

Updates #37608

Change-Id: I3fbae3344f1c95aff0a69c1e4f61ef637a54774e
Reviewed-on: https://go-review.googlesource.com/c/go/+/222917


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