cmd/compile: minor writebarrier cleanup

cmd/compile: minor writebarrier cleanup

This CL mainly moves some work to the switch on w.Op,
to make a follow-up change simpler and clearer.

Updates #19838

Change-Id: I86f3181c380dd60960afcc24224f655276b8956c
Reviewed-on: https://go-review.googlesource.com/42010
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>