Query if the register returned by GenerateCode must be freed.
For almost all Expression nodes freeRegister always returns true, which means the return register is temporarly allocated. Therefore it must be freed by the caller of GenerateCode. In some rare cases a register is allocated permenently in a scope (e.g. this), and freeRegister will return false. The use of this 'switch' allows generation of much more efficent code.
true - if register obtained by GenerateCode should be freed false - if register obtained by GenerateCode shouldn't be freed