Improve array to string conversion.

src/hydrogen-instructions.h

Index: src/hydrogen-instructions.h
diff --git a/src/hydrogen-instructions.h b/src/hydrogen-instructions.h
index 47a9da3b45deb917e7b4a5c213b4db57a3625d1d..74856c1d376ee93fb3151b8ef3920e53c30ddf4e 100644
--- a/src/hydrogen-instructions.h
+++ b/src/hydrogen-instructions.h
@@ -155,6 +155,7 @@ class LChunkBuilder;
   V(Return)                                    \
   V(Ror)                                       \
   V(Sar)                                       \
+  V(SeqStringSetChar)                          \
   V(Shl)                                       \
   V(Shr)                                       \
   V(Simulate)                                  \
@@ -5212,6 +5213,33 @@ class HDateField: public HUnaryOperation {
 };
 
 
+class HSeqStringSetChar: public HTemplateInstruction<3> {
+ public:
+  HSeqStringSetChar(bool is_one_byte,
+                    HValue* string,
+                    HValue* index,
+                    HValue* value) : is_one_byte_(is_one_byte) {
+    SetOperandAt(0, string);
+    SetOperandAt(1, index);
+    SetOperandAt(2, value);
+  }
+
+  bool is_one_byte() { return is_one_byte_; }
+  HValue* string() { return OperandAt(0); }
+  HValue* index() { return OperandAt(1); }
+  HValue* value() { return OperandAt(2); }
+
+  virtual Representation RequiredInputRepresentation(int index) {
+    return Representation::Tagged();
+  }
+
+  DECLARE_CONCRETE_INSTRUCTION(SeqStringSetChar)
+
+ private:
+  bool is_one_byte_;
+};
+
+
 class HDeleteProperty: public HBinaryOperation {
  public:
   HDeleteProperty(HValue* context, HValue* obj, HValue* key)