tcg/arm: Fix constraints for sub

tcg/arm: Fix constraints for sub

In 7536b82d288 we lost the rI constraint that allowed the use of
RSB to perform reg = imm - reg.  At the same time, drop support
for reg = reg - imm, which is now transformed generically to
addition, and need not be handled by the backend.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>