don't invalidate PN, rewrite of this code is in progress anyway.

llvm-svn: 86639
diff --git a/llvm/lib/Transforms/Scalar/JumpThreading.cpp b/llvm/lib/Transforms/Scalar/JumpThreading.cpp
index 46d40ef..7a7f407 100644
--- a/llvm/lib/Transforms/Scalar/JumpThreading.cpp
+++ b/llvm/lib/Transforms/Scalar/JumpThreading.cpp
@@ -223,10 +223,11 @@
       U = PNV;
       
       // See if we can simplify it.
-      if (Value *V = SimplifyInstruction(User, TD)) {
-        User->replaceAllUsesWith(V);
-        User->eraseFromParent();
-      }
+      if (User != PN)
+        if (Value *V = SimplifyInstruction(User, TD)) {
+          User->replaceAllUsesWith(V);
+          User->eraseFromParent();
+        }
     }
     
     PN->replaceAllUsesWith(PNV);