Store-store elimination can potentially allocate many temporary data
structures, all of which are contained in its so-called temporary
zone. The zone is 'temporary' in the sense of only existing for the
lifetime of the StoreStoreEliminationPhase; but all allocated memory
remains alive until the phase terminates.

Investigation of a pathological case (see the linked bug) showed this
zone reaching a size of 4GB. The cause in this specific case was the
UnobservableSet data structure, conceptually an immutable set (i.e.
each operation creates a full copy). This CL changes the
UnobservableSet to use a PersistentMap backing store rather than a
ZoneSet. PersistentMap is intended for exactly this use-case, since
copies are basically free and updates only change small parts of the
data structure.

Memory consumption for the linked pathological case drops from 4GB to
70MB and execution speeds up from 20s to 17s.

Bug: v8:9574
Change-Id: I902b27f1aa42b88ddd905ee941df24028a68a9bd
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1795351
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63819}