Convert the scheduling pass from scheduling backwards to scheduling forwards.
Forward scheduling makes it easier to prioritize scheduling values as
soon as they are ready, which is important for things like nil checks,
select ops, etc.
Forward scheduling is also quite a bit clearer. It was originally
backwards because computing uses is tricky, but I found a way to do it
simply and with n lg n complexity. The new scheme also makes it easy
to add new scheduling edges if needed.
Fixes #42673
Update #56568
Change-Id: Ibbb38c52d191f50ce7a94f8c1cbd3cd9b614ea8b
Reviewed-on: https://go-review.googlesource.com/c/go/+/270940
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: Keith Randall <khr@golang.org> Reviewed-by: David Chase <drchase@google.com>