This is a reland of 20f33823.
The GC crash should be fixed by https://crrev.com/c/1108208.

Original change's description:
> [wasm] Enable Liftoff by default on ia32 and x64
>
> R=titzer@chromium.org, hablich@chromium.org
>
> Bug: v8:6600, chromium:787421
> Change-Id: Ia8ae56ddef3b27b0721d5a66ff19abe098a2c6ca
> Reviewed-on: https://chromium-review.googlesource.com/1109899
> Reviewed-by: Ben Titzer <titzer@chromium.org>
> Reviewed-by: Michael Hablich <hablich@chromium.org>
> Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#53965}

TBR=titzer@chromium.org, hablich@chromium.org

Bug: v8:6600, chromium:787421
Change-Id: I2228084d7bd8ef8e96027bbd3dd81bc42d1dc533
Reviewed-on: https://chromium-review.googlesource.com/1112097Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53982}