FLAG_turboprop was used to test the turboprop compiler without any
further tierup to TurboFan. This cl changes:
- FLAG_turboprop to also tier up to TurboFan.
- Introduces FLAG_turboprop_as_toptier to continue running the
  configuration without tierup.
- Removes FLAG_turboprop_as_midtier which is same as FLAG_turboprop.

Bug: v8:9684
Change-Id: I487bda13d226434837770ecc43b3ced7c31ccf19
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2622214
Commit-Queue: Mythri Alle <mythria@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72101}