A64: Fix int32 use in Lithium string functions

src/a64/codegen-a64.h

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 20 matching lines @@
 #include "ast.h"
 #include "ic-inl.h"
 
 namespace v8 {
 namespace internal {
 
 class StringCharLoadGenerator : public AllStatic {
  public:
   // Generates the code for handling different string types and loading the
   // indexed character into |result|.  We expect |index| as untagged input and
-  // |result| as untagged output.
+  // |result| as untagged output. Register index is asserted to be a 32-bit W
+  // register.
   static void Generate(MacroAssembler* masm,
                        Register string,
                        Register index,
                        Register result,
                        Label* call_runtime);
 
  private:
   DISALLOW_COPY_AND_ASSIGN(StringCharLoadGenerator);
 };
 
 
 class MathExpGenerator : public AllStatic {
  public:
   static void EmitMathExp(MacroAssembler* masm,
                           DoubleRegister input,
                           DoubleRegister result,
                           DoubleRegister double_scratch1,
                           DoubleRegister double_scratch2,
                           Register temp1,
                           Register temp2,
                           Register temp3);
 
  private:
   DISALLOW_COPY_AND_ASSIGN(MathExpGenerator);
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_A64_CODEGEN_A64_H_