The {MaskOut} operation is faster than using {operator~}, since
{operator~} needs to ensure to return a valid register list, so it
contains an additional AND operation with the mask of all registers.
Hence use {MaskOut} more consistently, and remove the now unused
{operator~} on {LiftoffRegList}.

R=ahaas@chromium.org

Bug: v8:6600
Change-Id: Icf072e564c4e391cafa2abeeba53cf275cee75f8
Reviewed-on: https://chromium-review.googlesource.com/c/1275810Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56560}