target-arm: Handle UNDEF cases for Neon 3-regs-same insns

Correct the handling of UNDEF cases for the NEON "3 registers same
size" forms, by adding missing checks and rationalising some others
so they are done early enough to avoid leaking TCG temporaries.

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