Improve SeqStringSetChar implementation.

src/hydrogen-instructions.h

Index: src/hydrogen-instructions.h
diff --git a/src/hydrogen-instructions.h b/src/hydrogen-instructions.h
index f16efe6de83276be38aaf9b299f32586385761f0..832d3c2e3d8c099ccda9344fccb8ea81e6b88d5e 100644
--- a/src/hydrogen-instructions.h
+++ b/src/hydrogen-instructions.h
@@ -6266,6 +6266,7 @@ class HSeqStringSetChar: public HTemplateInstruction<3> {
     SetOperandAt(0, string);
     SetOperandAt(1, index);
     SetOperandAt(2, value);
+    set_representation(Representation::Tagged());
   }
 
   String::Encoding encoding() { return encoding_; }
@@ -6274,7 +6275,8 @@ class HSeqStringSetChar: public HTemplateInstruction<3> {
   HValue* value() { return OperandAt(2); }
 
   virtual Representation RequiredInputRepresentation(int index) {
-    return Representation::Tagged();
+    return (index == 0) ? Representation::Tagged()
+                        : Representation::Integer32();
   }
 
   DECLARE_CONCRETE_INSTRUCTION(SeqStringSetChar)