Saturating conversions are implemented on all platforms now, so remove
the (unreachable) bailout.
Also, move the new cases up to the other macros, such that the
indentation (by clang-format) makes more sense.

R=rstz@chromium.org, thibaudm@chromium.org

Bug: v8:10520
Change-Id: Ia44a4852a087b0a2c19b3d62aebd73c834fa9af9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2289982Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68782}