Fold away x === null comparisons when propagated cid of x is not kDynamicCid.

runtime/vm/intermediate_language.h

Index: runtime/vm/intermediate_language.h
diff --git a/runtime/vm/intermediate_language.h b/runtime/vm/intermediate_language.h
index 646eeba06740cdebf980f8e46251483b3d347a29..ccb0b2def5266170f6e7ddce541763bb2937540c 100644
--- a/runtime/vm/intermediate_language.h
+++ b/runtime/vm/intermediate_language.h
@@ -1177,6 +1177,7 @@ class PhiInstr : public Definition {
   // Phi is alive if it reaches a non-environment use.
   bool is_alive() const { return is_alive_; }
   void mark_alive() { is_alive_ = true; }
+  void mark_dead() { is_alive_ = false; }
 
   virtual Representation RequiredInputRepresentation(intptr_t i) const {
     return representation_;