target-arm: Simplify checking of size field in Neon 2reg-misc forms

Many of the Neon "2 register misc" instruction forms require invalid
size fields to cause the instruction to UNDEF. Pull this information
out into an array; this simplifies the code and also means we can do
the check early and avoid the problem of leaking TCG temporaries in
the illegal_op case.

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