Bring back raw SetAccumulator, instead of the separate
SetAccumulatorToNew/ExistingNode. SetAccumulator (and StoreRegister) are
now expected to only ever be called on new Nodes, with some DCHECKs
tracking which nodes are new guaranteeing this.

Bug: v8:7700
Change-Id: I5657fa85dc05445bc3d6956ebcd5541ec1cedfad
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3579362
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Victor Gomes <victorgomes@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79908}