tcg: Split out tcg_out_ext16u

tcg: Split out tcg_out_ext16u

We will need a backend interface for performing 16-bit zero-extend.
Use it in tcg_reg_alloc_op in the meantime.

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