Basic support for LICM of fully invariant loads.

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 e28e5aa919e0e2be2d5d458fedea18d7ddc9d046..2b6ad255e7708577e880a5f834af175601eb505f 100644
--- a/runtime/vm/intermediate_language.h
+++ b/runtime/vm/intermediate_language.h
@@ -618,7 +618,7 @@ class Instruction : public ZoneAllocated {
         previous_(NULL),
         next_(NULL),
         env_(NULL),
-        expr_id_(-1) { }
+        expr_id_(kNoExprId) { }
 
   virtual Tag tag() const = 0;
 
@@ -794,9 +794,11 @@ FOR_EACH_INSTRUCTION(INSTRUCTION_TYPE_CHECK)
   // Get the block entry for this instruction.
   virtual BlockEntryInstr* GetBlock() const;
 
-  // Id for instructions used in CSE.
+  // 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; }
 
   // Returns a hash code for use with hash maps.
   virtual intptr_t Hashcode() const;
@@ -888,6 +890,10 @@ FOR_EACH_INSTRUCTION(INSTRUCTION_TYPE_CHECK)
 
   virtual void RawSetInputAt(intptr_t i, Value* value) = 0;
 
+  enum {
+    kNoExprId = -1
+  };
+
   intptr_t deopt_id_;
   intptr_t lifetime_position_;  // Position used by register allocator.
   Instruction* previous_;