target/arm: fix TCG temp leak in aarch64 rev16

Fix a TCG temporary leak in the new aarch64 rev16 handling.

Signed-off-by: Emilio G. Cota <cota@braap.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
