We currently iterate the list of unexecuted register moves repeatedly,
always executing the moves whose destination register is not being used
as source register any more. This can lead to quadratic execution times
if only a small number of moves is processed in every iteration.

This CL refactors this such that we iterate the moves at most three
times: Once for executing moves which can be executed right away (fast
path) and for computing the source register use counts. A second time
to execute all remaining non-cyclic moves, and a third time to execute
cyclic moves.
During the second and third iteration, whenever we decrement the source
register use count, we check whether it drops to zero and execute the
respective move right away.

R=ahaas@chromium.org

Bug: v8:6600, v8:8423
Change-Id: I503328f5ae5f0208e35d53c71b4c289d75799892
Reviewed-on: https://chromium-review.googlesource.com/c/1397703
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58588}