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}