Create tcg/tcg-op-common.h, moving everything that does not concern
TARGET_LONG_BITS or TCGv.  Adjust tcg/*.c to use the new header
instead of tcg-op.h, in preparation for compiling tcg/ only once.

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