target-arm: Use lookup table for size check on Neon 3-reg-same insns

Simplify the checks for invalid size values for the Neon "three registers
of the same size" instruction forms (and add them where they were missing)
by using a lookup table.

This includes adding symbolic constants for the op values in this space,
since we now use them in multiple places.

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