Simplify accounting for compilation progress. Instead of complicated
logic in {OnUnitsFinished} the compilation progress is initialized in
{InitializeCompilationProgress}. We now keep tack of
  - the required baseline tier,
  - the required top tier, and
  - the currently reached tier.
With this information {OnUnitsFinished} determines whether baseline and
top tier compilation are completed.

Bug: v8:9003
Change-Id: I3d147613f30363aade9ad5bf65be6e4d105e561e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1583722
Commit-Queue: Frederik Gossen <frgossen@google.com>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61019}