tcg: Introduce tcg_out_movext

tcg: Introduce tcg_out_movext

This is common code in most qemu_{ld,st} slow paths, extending the
input value for the store helper data argument or extending the
return value from the load helper.

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