Add operand support to the Instruction base class. Add setOperand methods

to all the things.  Fill out the OneOperand trait class with support for
getting and setting operands, allowing DimOp to have a working
get/setOperand() method.

I'm not thrilled with the extra template argument on OneOperand, I'll will
investigate removing that in a follow-on patch.

PiperOrigin-RevId: 205679696