Precursor to moving new_value from the global state to DisasContext

USR will need to stay in the global state because some helpers will
set it's value

Signed-off-by: Taylor Simpson <tsimpson@quicinc.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230427230012.3800327-17-tsimpson@quicinc.com>