This is a reland of c2aaf0a6

Original change's description:
> [wasm][liftoff] Optimize one-armed ifs
> 
> Do not implement one-armed ifs by emulating an empty else branch. In
> Liftoff, we can generate better code and save compile time by handling
> this specially. If the merge point at the end of the if is not reached
> by the if-branch, we do not need to generate any merge code.
> 
> R=titzer@chromium.org
> 
> Bug: v8:6600, v8:8423
> Change-Id: Ie8ea69dd7491f225605a8e1b986d275d869aa90b
> Reviewed-on: https://chromium-review.googlesource.com/c/1356508
> Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
> Reviewed-by: Ben Titzer <titzer@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#57968}

Bug: v8:6600, v8:8423
Change-Id: I6d5eea9f860486768779a33bf6bd7b87cbfc2af0
Reviewed-on: https://chromium-review.googlesource.com/c/1361040Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58024}