Coalescer should not delete copy instructions whose defs are partially dead. e.g.

%RDI<def,dead> = MOV64rr %RAX<kill>, %EDI<imp-def>


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@100804 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/MachineInstr.h

@@ -357,6 +357,10 @@ public:
   /// return 0.
   unsigned isConstantValuePHI() const;
 
+  /// allDefsAreDead - Return true if all the defs of this instruction are dead.
+  ///
+  bool allDefsAreDead() const;
+
   //
   // Debugging support
   //