target-arm: Handle UNDEF cases for Neon invalid modified-immediates

For Neon "one register and a modified immediate value" forms, the
combination op=1 cmode=1111 is unallocated and should UNDEF.
All instructions of this form also UNDEF if Q == 1 and Vd<0> == 1.
We also add a comment on the only UNPREDICTABLE in this space.

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