Request: Make SIMDRegister a literal type

Since JUCE is C++11 now, it would be useful for SIMDRegister to be a literal type so we can make constexpr values and constexpr functions that return a SIMDRegister.

I think all that needs to change are that the constructors need to be marked constexpr and the destructor removed.

Orrrrr maybe it’s not possible and I need to read more about literal types.