target/ppc: introduce separate VSX_CMP macro for xvcmp* instructions

target/ppc: introduce separate VSX_CMP macro for xvcmp* instructions

Rather than perform the VSR register decoding within the helper itself,
introduce a new VSX_CMP macro which performs the decode based upon xT, xA
and xB at translation time.

Subsequent commits will make the same changes for other instructions however
the xvcmp* instructions are different in that they return a set of flags to be
optionally written back to the crf[6] register. Move this logic from the
helper function to the generator function, along with the float_status update.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20190616123751.781-5-mark.cave-ayland@ilande.co.uk>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>