This adds support for direct calls in Liftoff.

Drive-by: Fix / extend two tests for calls which were helpful for
developing this CL.

R=ahaas@chromium.org

Bug: v8:6600
Change-Id: I20a98d9dd330da9a020c8c9b5c10b04e94af684d
Reviewed-on: https://chromium-review.googlesource.com/847579
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50427}