This is a reland of 6c613288
Original change's description:
> [Liftoff] Implement parallel register moves
> 
> This was not implemented so far, leading to lots of clusterfuzz issues.
> Testing this feature would require complicating the interface a lot and
> exposing more implementation details in the header file, hence we just
> go with regression tests for now.
> 
> R=ahaas@chromium.org, titzer@chromium.org
> 
> Bug: v8:6600, chromium:782280
> Change-Id: I12863f3eb59a8dffdcc7d3bfb8e1f0ae0eec15ee
> Reviewed-on: https://chromium-review.googlesource.com/758772
> Reviewed-by: Andreas Haas <ahaas@chromium.org>
> Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#49286}

Bug: v8:6600, chromium:782280
Change-Id: I82a75bfeaf83dc63a2917da3ccdc4721c5d689e7
Reviewed-on: https://chromium-review.googlesource.com/763387Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49292}