target-arm: Handle UNDEF cases for Neon 2 regs + scalar forms

Add missing checks for cases which must UNDEF in the Neon "2 registers and
a scalar" data processing instruction space.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
