For finching Liftoff on mobile, we want to support Liftoff-only,
TurboFan-only, and Liftoff with tier-up to TurboFan. Hence, chrome needs
to be able to set the liftoff flag separately from the tier-up flag, and
there should not be an implication, since that implication would be
applied before all flags have been set to their final state.
See https://crrev.com/c/1988360 for the chromium-side change.

Note that this CL does not change any defaults.
After this change, Liftoff can be disabled by passing '--no-liftoff',
independent of the tier-up flag (this would be a TurboFan-only
configuration).
A Liftoff-only version would specify --liftoff and --no-wasm-tier-up.
The default on Desktop (--liftoff and --wasm-tier-up) compiles with
Liftoff first, and tiers up to TurboFan.

R=ahaas@chromium.org

Bug: chromium:1040061
Change-Id: I0291ec20f5a29f3a9e23c25c93907d1d8ffabd51
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1988548Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65635}