target-arm: Handle UNDEF cases for Neon 3-regs-different-widths

Add missing UNDEF checks for instructions in the Neon "3 registers of
different widths" data processing space.

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