Focuses on decoder implementation and unittests of decoding return call instructions

Bug: v8:7431
Change-Id: Ib1351bb26f8bac0a766d633486492fcd8ead627b
Reviewed-on: https://chromium-review.googlesource.com/c/1455476
Commit-Queue: Francis McCabe <fgm@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59582}