cmov is often much faster than branches (in particular if the branches
cannot be predicted). It also has a much more predictable performance,
and is shorter.
This CL uses cmov on x64 for now. Other platforms (maybe using other
instructions than cmov) can be added later.

R=thibaudm@chromium.org

Bug: v8:10740
Change-Id: Ifab3d570b8eea784376e1f768d6ba3828efcc01a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2315978Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69027}