cmd/compile: reorganize prove pass domain relation table

cmd/compile: reorganize prove pass domain relation table

Move some code from when we learn that we take a branch, to when
we learn that a boolean is true or false. It is more consistent
this way (and may lead to a few more cases where we can derive
useful relations).

Change-Id: Iea7b2d6740e10c9c71c4b1546881f501da81cd21
Reviewed-on: https://go-review.googlesource.com/c/go/+/599098
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>