cmd/compile: clean up prove pass

cmd/compile: clean up prove pass

flowLimit no longer needs to return whether it updated any information,
after CL 714920 which got rid of the iterate-until-done paradigm.

Change-Id: I0c5f592578ff27c27586c1f8b8a8d9071d94846d
Reviewed-on: https://go-review.googlesource.com/c/go/+/720720
Reviewed-by: Youlin Feng <fengyoulin@live.com>
Reviewed-by: Mark Freeman <markfreeman@google.com>
Reviewed-by: Jorropo <jorropo.pgm@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>