This reverts commit b3d748a2.

Reason for revert: Regressions, see https://crbug.com/1023423.

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}

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

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

Bug: v8:9574
Change-Id: I5d684198f9c4575a0c892076459cc2c20dce9aec
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1910944Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64912}