Instead of unconditionally exiting to the exec loop, use the
lookup_and_goto_ptr helper to jump to the target if it is valid.

Perf impact: see next commit's log.
Reviewed-by: NAlex Bennée <alex.bennee@linaro.org>
Reviewed-by: NRichard Henderson <rth@twiddle.net>
Signed-off-by: NEmilio G. Cota <cota@braap.org>
Message-Id: <1493263764-18657-7-git-send-email-cota@braap.org>
Signed-off-by: NRichard Henderson <rth@twiddle.net>