This reverts commit a9ea67d4.

Reason for revert: Regressions https://crbug.com/1025160.

Original change's description:
> Reland "[regalloc] Use an adaptive data structure for live sets"
>
> This is a reland of b3d748a2
>
> Original change's description:
> > [regalloc] Use an adaptive data structure for live sets
> >
> > 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/+/1891693
> > Reviewed-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}
>
> Bug: v8:9574
> Change-Id: I5a95d56c33a98cc5c6c58ff9308314e2eefa462c
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1910953
> Reviewed-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@{#64950}

TBR=jgruber@chromium.org,tebbi@chromium.org,thibaudm@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: v8:9574,chromium:1025160
Change-Id: I177d64eed588cd09c999e15b04d37630c2c6538b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1918255
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64998}