[RegBankSelect] Add helper class for repairing code placement.

When assigning the register banks we may have to insert repairing code
to move already assigned values accross register banks.

Introduce a few helper classes to keep track of what is involved in the
repairing of an operand:
- InsertPoint and its derived classes record the positions, in the CFG,
  where repairing has to be inserted.
- RepairingPlacement holds all the insert points for the repairing of an
  operand plus the kind of action that is required to do the repairing.

This is going to be used to keep track of how the repairing should be
done, while comparing different solutions for an instruction. Indeed, we
will need the repairing placement to capture the cost of a solution and
we do not want to compute it a second time when we do the actual
repairing.

llvm-svn: 270167