This removes the need to define the derived Operand class before the derived
Value class. The major benefit of this refactoring is that we no longer need
the OpaqueValue class, as OpOperand can now be defined after Value. As part of
this refactoring the BlockOperand and OpOperand classes are moved out of
UseDefLists.h and to more suitable locations in BlockSupport and Value. After
this change, UseDefLists.h is almost entirely composed of generic use def utilities.
Differential Revision: https://reviews.llvm.org/D103353
c89dff5855