[turbofan] Clean up some more things in the serializer

- Move {function_} and {closure_hints_} out of each {Environment}, as
  it's sufficient to only have it in the serializer class.
- Similarly, remove {parameter_count_} and {register_count_} from
  {Environment}. To accomodate for that, change implementation of
  "dead" environments to be based on a flag.
- Remove {ClearEphemeralHints}. This was called for any bytecode not
  recognized by the serializer. I think it makes more sense to just
  do nothing at all for those, i.e. keep whatever hints we have.
- Split the magical {ephemeral_hints_} vector into three separate
  things for clarity: {accumulator_hints_}, {parameters_hints_}, and
  {locals_hints}. Note that we already had {current_context_hints_}.

Bug: v8:7790
Change-Id: Icbe33f500bdbd0f8002570934f59425f49c9de12
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1958010
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65400}