target/arm: Use tcg_constant_i32() in gen_rev16()

Since the mask is a constant value, use tcg_constant_i32()
instead of a TCG temporary.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20211029231834.2476117-6-f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>