Refactor load forwarding pass to use a Place abstraction.

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 94da04591a7490975cafbeb80f00eb6c4d8e7e15..5c5f2617ef9bbe0e6d04d29b858a03ef1de7de87 100644
--- a/runtime/vm/intermediate_language.h
+++ b/runtime/vm/intermediate_language.h
@@ -640,7 +640,7 @@ class Instruction : public ZoneAllocated {
         previous_(NULL),
         next_(NULL),
         env_(NULL),
-        expr_id_(kNoExprId) { }
+        place_id_(kNoPlaceId) { }
 
   virtual Tag tag() const = 0;
 
@@ -731,6 +731,7 @@ class Instruction : public ZoneAllocated {
   virtual const char* DebugName() const = 0;
 
   // Printing support.
+  const char* ToCString() const;
   virtual void PrintTo(BufferFormatter* f) const;
   virtual void PrintOperandsTo(BufferFormatter* f) const;
 
@@ -823,11 +824,10 @@ FOR_EACH_INSTRUCTION(INSTRUCTION_TYPE_CHECK)
   // Get the block entry for this instruction.
   virtual BlockEntryInstr* GetBlock() const;
 
-  // Id for load instructions used during load forwarding pass and later in
-  // LICM.
-  intptr_t expr_id() const { return expr_id_; }
-  void set_expr_id(intptr_t expr_id) { expr_id_ = expr_id; }
-  bool HasExprId() const { return expr_id_ != kNoExprId; }
+  // Place identifiers used by the load optimization pass.
+  intptr_t place_id() const { return place_id_; }
+  void set_place_id(intptr_t place_id) { place_id_ = place_id; }
+  bool HasPlaceId() const { return place_id_ != kNoPlaceId; }
 
   // Returns a hash code for use with hash maps.
   virtual intptr_t Hashcode() const;
@@ -927,7 +927,7 @@ FOR_EACH_INSTRUCTION(INSTRUCTION_TYPE_CHECK)
   virtual void RawSetInputAt(intptr_t i, Value* value) = 0;
 
   enum {
-    kNoExprId = -1
+    kNoPlaceId = -1
   };
 
   intptr_t deopt_id_;
@@ -935,7 +935,7 @@ FOR_EACH_INSTRUCTION(INSTRUCTION_TYPE_CHECK)
   Instruction* previous_;
   Instruction* next_;
   Environment* env_;
-  intptr_t expr_id_;
+  intptr_t place_id_;
 
   DISALLOW_COPY_AND_ASSIGN(Instruction);
 };
@@ -1379,6 +1379,7 @@ class JoinEntryInstr : public BlockEntryInstr {
   void RemoveDeadPhis(Definition* replacement);
 
   void InsertPhi(PhiInstr* phi);
+  void RemovePhi(PhiInstr* phi);
 
   virtual void PrintTo(BufferFormatter* f) const;
 
@@ -1701,6 +1702,7 @@ 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_;