Wasm tier-up first compiles the whole module using Liftoff, and then
using Turbofan. The idea is to achieve fast start-up times by first
running Liftoff-compiled code. In the meantime we finish compilation
with Turbofan, and replace the Liftoff-compiled code as soon
as Turbofan finished compilation, thus achieving high performance.
Tier-up is enabled through the flag FLAG_wasm_tier_up.

Bug: v8:6600
Change-Id: I70552969c53d909a591666a1e7ce1ee1419b2f34
Reviewed-on: https://chromium-review.googlesource.com/1010422
Commit-Queue: Kim-Anh Tran <kimanh@google.com>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52759}