Aggressively prune the DFS when inserting phi-defs.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@111394 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/SplitKit.cpp

@@ -446,9 +446,9 @@ VNInfo *LiveIntervalMap::mapValue(const VNInfo *ParentVNI, SlotIndex Idx) {
     }
 
     // No need to search the children, we found a dominating value.
-    // FIXME: We could prune up to the last phi-def we inserted, need df_iterator
-    // for that.
-    IDFI.skipChildren();
+    // MBB is either the found dominating value, or the last phi-def we created.
+    // Either way, the children of MBB would be shadowed, so don't search them.
+    IDFI.skipChildren(MBB);
   }
 
   // The search should at least find a dominating value for IdxMBB.