Add a simple forward check elimination based on a side hashmap of "known
node aspects", namely the node type and node map (if any). This set of
aspects is cloned when merge states are created, and destructively
merged when merged into existing merge states -- destructive cloning
here means removing any mismatching information. This allows information
in dominators to be preserved.

Maps are kept separate from node types because we want to distinguish
between stable and unstable maps, where the former need a dependency and
the latter must be flushed across side-effecting calls.

The representation of this known information is currently very
inefficient, and won't win us any compilation speed prizes -- just
ZoneMaps keyed on ValueNode*. We should optimize this to take into
account some sort of liveness information, and clear out nodes that
aren't reachable anymore. There is also a lot more information we could
store per Node, e.g. known loaded fields or alternative representations;
depending on what we want to store and how that has to be invalidated,
we likely might need an alternative way of representing it. This
implementation is good enough for now though, for measuring the impact
of check elimination.

Bug: v8:7700
Change-Id: I2f001dedf8ab5d86f8acaa22416617bd80701982
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3865160
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Victor Gomes <victorgomes@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Jakob Linke <jgruber@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82911}