Introduce a common base class (IROperandOwner) between Instruction and

Statement, which paves the way to make SSAValue's have a useful owner
available, which will allow subsequent patches to improve their use/def
chains.

While I'm poking at this, shrink sizeof(Instruction) and sizeof(Statement) by a
word by packing the kind and location together into a single PointerIntPair.

NFC.

PiperOrigin-RevId: 218959651