cmd/compile: use proper work queue for escape graph walking

The old escape analysis code used to repeatedly walk the entire flow
graph until it reached a fixed point. With escape.go, I wanted to
avoid this if possible, so I structured the walking code with two
constraints:

1. Always walk from the heap location last.

2. If an object escapes, ensure it has flow edge to the heap location.

This works, but it precludes some graph construction
optimizations. E.g., if there's an assignment "heap = &x", then we can
immediately tell that 'x' escapes without needing to visit it during
the graph walk. Similarly, if there's a later assignment "x = &y", we
could immediately tell that 'y' escapes too. However, the natural way
to implement this optimization ends up violating the constraints
above.

Further, the constraints above don't guarantee that the 'transient'
flag is handled correctly. Today I think that's handled correctly
because of the order that locations happen to be constructed and
visited based on the AST, but I've felt uneasy about it for a little
while.

This CL changes walkAll to use a proper work queue (technically a work
stack) to track locations that need to be visited, and allows walkOne
to request that a location be re-visited.

Passes toolstash-check.

Change-Id: Iaa6f4d3fe4719c04d67009fb9a2a3e4930b3d7c2
Reviewed-on: https://go-review.googlesource.com/c/go/+/196958


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>