Improve array to string conversion.

src/x64/lithium-x64.h

Index: src/x64/lithium-x64.h
diff --git a/src/x64/lithium-x64.h b/src/x64/lithium-x64.h
index b5d435b5214ad84672d081d279286782dc824c62..6b886a893f0276910ab9740815feb5c308184c88 100644
--- a/src/x64/lithium-x64.h
+++ b/src/x64/lithium-x64.h
@@ -149,6 +149,7 @@ class LCodeGen;
   V(Random)                                     \
   V(RegExpLiteral)                              \
   V(Return)                                     \
+  V(SeqStringSetChar)                           \
   V(ShiftI)                                     \
   V(SmiTag)                                     \
   V(SmiUntag)                                   \
@@ -1155,6 +1156,30 @@ class LDateField: public LTemplateInstruction<1, 1, 0> {
 };
 
 
+class LSeqStringSetChar: public LTemplateInstruction<1, 3, 0> {
+ public:
+  LSeqStringSetChar(bool is_one_byte,
+                    LOperand* string,
+                    LOperand* index,
+                    LOperand* value) : is_one_byte_(is_one_byte) {
+    inputs_[0] = string;
+    inputs_[1] = index;
+    inputs_[2] = value;
+  }
+
+  LOperand* string() { return inputs_[0]; }
+  LOperand* index() { return inputs_[1]; }
+  LOperand* value() { return inputs_[2]; }
+  bool is_one_byte() { return is_one_byte_; }
+
+  DECLARE_CONCRETE_INSTRUCTION(SeqStringSetChar, "seq-string-set-char")
+  DECLARE_HYDROGEN_ACCESSOR(SeqStringSetChar)
+
+ private:
+  bool is_one_byte_;
+};
+
+
 class LThrow: public LTemplateInstruction<0, 1, 0> {
  public:
   explicit LThrow(LOperand* value) {