HPhis with HParameter inputs must have Tagged representation

src/hydrogen-instructions.h

Index: src/hydrogen-instructions.h
diff --git a/src/hydrogen-instructions.h b/src/hydrogen-instructions.h
index 810a2fdd520d26e0d636e97ee470681210f293ac..a610fdb94732088c81c3ce01a3f1b61564a3fea1 100644
--- a/src/hydrogen-instructions.h
+++ b/src/hydrogen-instructions.h
@@ -5312,6 +5312,12 @@ class HParameter FINAL : public HTemplateInstruction<0> {
     return Representation::None();
   }
 
+  virtual Representation KnownOptimalRepresentation() OVERRIDE {
+    // If a parameter is an input to a phi, that phi should not
+    // choose any more optimistic representation than Tagged.
+    return Representation::Tagged();
+  }
+
   DECLARE_CONCRETE_INSTRUCTION(Parameter)
 
  private: