.. with readability and simplicity in mind.

- Rename OptimizationMarker to the (shorter) TieringState. 'Tiering'
  also matches 'TieringManager' terminology.
- Rename the values:
  kNone -> kNone
  kInOptimizationQueue -> kInProgress
  kCompileFoo_NotConcurrent -> kRequestFoo_Synchronous
  kCompileFoo_Concurrent -> kRequestFoo_Concurrent
- Likewise rename ConcurrencyMode::kNotConcurrent to kSynchronous.
- Add predicates to test enum values.
- Consistent lower case names for accessors on JSFunction and
  FeedbackVector.
- Instead of having to call HasOptimizationMarker() before using any
  other accessor, simply have optimization_marker() return kNone if
  no feedback vector exists.
- Drive-by: Enable the Unreachable() in MaybeOptimizeCode()
  unconditionally - this should never happen, there's no reason not
  to protect against this in release builds as well.

Bug: v8:12161
Change-Id: I67c03e2b7bd0a6b86d0c64f504ad8cb47e9e26ae
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3555774Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Jakob Linke <jgruber@chromium.org>
Auto-Submit: Jakob Linke <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79669}