cmd/compile: detect write barrier completion differently

cmd/compile: detect write barrier completion differently

Instead of keeping track of in which blocks write barriers complete,
introduce a new op that marks the exact memory state where the
write barrier completes.

For future use. This allows us to move some of the write barrier code
to between the start of the merging block and the WBend marker.

Change-Id: If3809b260292667d91bf0ee18d7b4d0eb1e929f0
Reviewed-on: https://go-review.googlesource.com/c/go/+/447777
Reviewed-by: Keith Randall <khr@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Keith Randall <khr@golang.org>