Simplify LiveDebugVariables by storing MachineOperand copies locations instead

of using a Location class with the same information.

When making a copy of a MachineOperand that was already stored in a
MachineInstr, it is necessary to clear the parent pointer on the copy. Otherwise
the register use-def lists become inconsistent.

Add MachineOperand::clearParent() to do that. An alternative would be a custom
MachineOperand copy constructor that cleared ParentMI. I didn't want to do that
because of the performance impact.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@123109 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/MachineOperand.h

@@ -153,6 +153,16 @@ public:
   MachineInstr *getParent() { return ParentMI; }
   const MachineInstr *getParent() const { return ParentMI; }
 
+  /// clearParent - Reset the parent pointer.
+  ///
+  /// The MachineOperand copy constructor also copies ParentMI, expecting the
+  /// original to be deleted. If a MachineOperand is ever stored outside a
+  /// MachineInstr, the parent pointer must be cleared.
+  ///
+  /// Never call clearParent() on an operand in a MachineInstr.
+  ///
+  void clearParent() { ParentMI = 0; }
+
   void print(raw_ostream &os, const TargetMachine *TM = 0) const;
 
   //===--------------------------------------------------------------------===//