cmd/compile: refactor and cleanup of common code/workaround

There's a glitch in how attributes from procs that do not
generate code are combined, and the workaround for this
glitch appeared in two places.

"One big pile is better than two little ones."

Updates #25426.

Change-Id: I252f9adc5b77591720a61fa22e6f9dda33d95350
Reviewed-on: https://go-review.googlesource.com/113717


Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>