Fix the value_type of defusechain_iterator to match its operator*()

defusechain_iterator has an operator*() and operator->() that return
references to a MachineOperand, but its "reference" and "pointer"
typedefs are set as if the iterator returns a MachineInstr reference.
This causes compilation errors when defusechain_iterator is used in
generic code that uses the "reference" and "pointer" typedefs.
This patch fixes this by updating the typedefs to use MachineOperand
instead of MachineInstr.

Reviewed By: mkitzan

Differential Revision: https://reviews.llvm.org/D97522