Reland of [turbofan] Implicitly emit eager checkpoint at graph building.

src/compiler/checkpoint-elimination.h

Index: src/compiler/checkpoint-elimination.h
diff --git a/src/compiler/checkpoint-elimination.h b/src/compiler/checkpoint-elimination.h
index 4d6aadac2f6481c530fb6bab9a364470c3407048..deb4c38b011619f1b0c4b0d4d34fff9cf29b3809 100644
--- a/src/compiler/checkpoint-elimination.h
+++ b/src/compiler/checkpoint-elimination.h
@@ -18,6 +18,10 @@
   ~CheckpointElimination() final {}
 
   Reduction Reduce(Node* node) final;
+
+ private:
+  Reduction ReduceCheckpoint(Node* node);
+  Reduction ReduceReturn(Node* node);
 };
 
 }  // namespace compiler