Fix possible self assign issue for DIEValue

In DIEValue's operator assignment constructor, it didn't identify if
the two obj is the same.
I add code to identify them so that it will work correctly when we do
self assign here.

Reviewed By: skan

Differential Revision: https://reviews.llvm.org/D150020

llvm/include/llvm/CodeGen/DIE.h

@@ -464,6 +464,8 @@ class DIEValue {
   }
 
   DIEValue &operator=(const DIEValue &X) {
+    if (this == &X)
+      return *this;
     destroyVal();
     Ty = X.Ty;
     Attribute = X.Attribute;