Rework inherentance hierarchy: Operation now derives from Statement, and

OperationInst derives from it.  This allows eliminating some forwarding
functions, other complex code handling multiple paths, and the 'isStatement'
bit tracked by Operation.

This is the last patch I think I can make before the big mechanical change
merging Operation into OperationInst, coming next.

This is step 15/n towards merging instructions and statements, NFC.

PiperOrigin-RevId: 227077411