This fixes a problem where ICs for transitioning stores go MEGAMORPHIC
if the transition target map dies in between invocations of the IC,
which is totally possible, since we only hold on weakly to these
transition targets (both from the FeedbackVectors and also from the
TransitonArrays).

The root problem here was an inconsistency in how the maps and handlers
are being reported by the FeedbackVector. On the on hand side the method
FeedbackVector::ExtractMaps() will report all receiver maps that are
still present (i.e. which haven't died themselves), but then the other
method FeedbackVector::FindHandlers() will only report handlers that are
still alive (i.e. which in case of transition target maps being used as
handlers haven't died yet). If the length of these lists don't match the
IC chickens out and goes MEGAMORPHIC. But this is exactly the case with
the transitioning stores, where there's no handler anymore, i.e. as can
be seen in this simple example:

```
// Flags: --expose-gc
function C() { this.x = 1; }
new C();
new C();
gc();     // map with the `C.x` property dies
new C();  // now the STORE_IC in C goes MEGAMORPHIC
```

So the problem is that we have these two methods that don't agree with
each other. Now FeedbackVector::ExtractMaps() is also used by TurboFan
and it even reports receiver maps for PREMONOMORPHIC state, which is
different from the use case that the ICs need. So I replaced the
FeedbackVector::FindHandlers() with a completely new method
FeedbackVector::ExtractMapsAndHandlers(), which returns both the maps
and handlers, exactly as the ICs need it. And only returns pairs for
which both the receiver map and the handler are still alive.

This fixes the odd problem that sometimes STORE_ICs going MEGAMORPHIC
for no apparent reason. Due to the weakness of the transition target
maps, they can still die and cause deoptimizations, but at least
TurboFan will now be able to reoptimize again later with the new maps
and still generate proper code.

Bug: v8:9316
Cq-Include-Trybots: luci.chromium.try:linux-rel,win7-rel
Change-Id: I74c8b60f792f310dc813f997e69efe9ad434296a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1637878
Auto-Submit: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61948}