cmd/compile: move raw writes out of write barrier code

cmd/compile: move raw writes out of write barrier code

Previously, the write barrier calls themselves did the actual
writes to memory. Instead, move those writes out to a common location
that both the wb-enabled and wb-disabled code paths share.

This enables us to optimize the write barrier path without having
to worry about performing the actual writes.

Change-Id: Ia71ab651908ec124cc33141afb52e4ca19733ac6
Reviewed-on: https://go-review.googlesource.com/c/go/+/447780
Reviewed-by: Michael Knyszek <mknyszek@google.com>
TryBot-Bypass: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>