This cleans up several parts of the tiering logic.
1) Instead of using the {ExecutionTier} to specify whether we do tier up
   or down, we introduce a new {TieringState} enum and use that
   consistently (also where a {bool} was used before).
2) When tiering up or tiering down, always recompile all functions. It's
   very unlikely that we can reuse previous code anyway (tiering down is
   cheap enough to just always do it, and when tiering up we need to
   recompile everything anyway).
3) Remove the {WasmEngine::RecompileAllFunctions} method and inline the
   implementation into callers.
4) Drive-by: Remove some obsolete comments and fix or extend others.

R=thibaudm@chromium.org

Bug: v8:10410
Change-Id: Ic765c6760dd97473ccfd469f22a2514695075587
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2151355Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67175}