[release-branch.go1.7] cmd/compile: escape analysis needs to run "flood" to fixed...

[release-branch.go1.7] cmd/compile: escape analysis needs to run "flood" to fixed point

In some cases the members of the root set from which flood
runs themselves escape, without their referents being also
tagged as escaping.  Fix this by reflooding from those roots
whose escape increases, and also enhance the "leak" test to
include reachability from a heap-escaped root.

Fixes #17318.

Change-Id: Ied1e75cee17ede8ca72a8b9302ce8201641ec593
Reviewed-on: https://go-review.googlesource.com/30693
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-on: https://go-review.googlesource.com/31290
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: David Chase <drchase@google.com>