On arm64, we had {RegisterBase::bit} (defined in assembler.h) and
{CPURegister::bit} (defined in assembler-arm.h). {CPURegister} inherits
from {RegisterBase}. The two methods methods have different
behaviour on the special {no_reg}, which is only relied on in very few
places.
This CL fixes these places to avoid the use of {no_reg}, and removes
the overwritten method.

R=mstarzinger@chromium.org
CC=​rodolph.perfetta@arm.com

Change-Id: I859cc0d4ffc48fae018ee262f3e5403774db87a8
Reviewed-on: https://chromium-review.googlesource.com/1042188Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Rodolph Perfetta <rodolph.perfetta%arm.com@gtempaccount.com>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52977}