RISC-V: MaxVectorSize is not consistently checked in several situations

Current logic will not check whether (MaxVectorSize < 16), after the assignment `MaxVectorSize = _initial_vector_length;`, in following situation.
a) if FLAG_IS_DEFAULT(MaxVectorSize) == true
b) if FLAG_IS_DEFAULT(MaxVectorSize) == false and (MaxVectorSize >= 16) and is_power_of_2(MaxVectorSize) and (MaxVectorSize > _initial_vector_length)

And in original code, the logic is not consistent for the situations between MaxVectorSize < 16 and MaxVectorSize >= 16, when is_power_of_2(MaxVectorSize) == false; for the former (<16) it's to disable RVV, for the latter (>=16) it's vm_exit.