target-arm: UNDEF on a VCVTT/VCVTB UNPREDICTABLE to avoid TCG assert

VCVTT/VCVTB with bit 8 set is UNPREDICTABLE; we choose to UNDEF.
This avoids a TCG assert later when the VCVTT/VCVTB code tries to
use a source register that wasn't ever set up.

We pull the check for the presence of the half-precision extension
up in to this common code as well.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
