Introduce a new GEN_VSX_HELPER_VSX_MADD macro for the generator function which
enables the source and destination registers to be decoded at translation time.

This enables the determination of a or m form to be made at translation time so
that a single helper function can now be used for both variants.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20190616123751.781-16-mark.cave-ayland@ilande.co.uk>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>