The {CountClearHalfWords} method is called whenever loading a constant
into a register. It showed up with >0.5% in Liftoff compilation
profiles. This CL refactors the method to return the number of *set*
halfwords instead of *cleared* halfwords and avoids the loop in the
implementation. This makes the method roughly twice as fast, and makes
the code more readable.

R=zhin@chromium.org

Bug: v8:11879
Change-Id: I7da8160b3c045e5fc1e97fc0e575083b3920cb5b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3151962Reviewed-by: Zhi An Ng <zhin@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#76771}