Live sets represent sets of live virtual registers at block entry and
exit points. They are usually sparsely populated; for example, a sample
taken from Octane2 shows 80% of sampled live sets with a fill ratio of
10% or less.

Prior to this CL, live sets were implemented as a statically-sized bit
vector. This is fine for low-ish virtual register counts, but becomes
wasteful at higher numbers.

This CL attempts to address this issue through an adaptive
implementation. Small live sets remain bit vectors, while larger sets
switch to a PersistentMap-based implementation. PersistentMap has very
memory-efficient add/remove/copy operations.

Of course, with adaptive data structures we enter the territory of
parameter fiddling. In this case, two parameters are used:
kMaxSmallSetSize controls when to switch implementations, and
kMaxDeletionsBeforePrune controls when pruning (= managing the # of
deleted entries in the map) sets in.

On the (degenerate) test case from the linked bug, the register
allocation zone shrinks from 1008MB to 475MB. For more realistic cases
I expect savings on the order of 10s of KB.

Bug: v8:9574
Change-Id: Id903bbe23f030b418e8d887ef4839c8d65126c52
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1891693Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64872}