There's semantically-but-not-literally equivalent code in
two cases for joining blocks' value lists in ssa/fuse.go.
It can be made literally equivalent, then commoned up.

Updates #25426.

Change-Id: Id1819366c9d22e5126f9203dcd4c622423994110
Reviewed-on: https://go-review.googlesource.com/113719
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>