Add code generation functions for data movement between
TCGv_i128 (mov) and to/from TCGv_i64 (concat, extract).

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