[regexp] Address various minor nits

src/code-stub-assembler.cc

 // Copyright 2016 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 #include "src/code-stub-assembler.h"
 #include "src/code-factory.h"
 #include "src/frames-inl.h"
 #include "src/frames.h"
 
 namespace v8 {
 namespace internal {
@@ skipping 3136 matching lines @@
       Int32LessThanOrEqual(instance_type,
                            Int32Constant(LAST_FIXED_TYPED_ARRAY_TYPE)));
 }
 
 Node* CodeStubAssembler::IsJSRegExp(Node* object) {
   return HasInstanceType(object, JS_REGEXP_TYPE);
 }
 
 Node* CodeStubAssembler::StringCharCodeAt(Node* string, Node* index,
                                           ParameterMode parameter_mode) {
+  if (parameter_mode == SMI_PARAMETERS) CSA_ASSERT(this, TaggedIsSmi(index));
   CSA_ASSERT(this, IsString(string));
 
   // Translate the {index} into a Word.
   Node* const int_index = ParameterToWord(index, parameter_mode);
+  CSA_ASSERT(this, IntPtrGreaterThanOrEqual(int_index, IntPtrConstant(0)));
 
   VARIABLE(var_result, MachineRepresentation::kWord32);
 
   Label out(this, &var_result), runtime_generic(this), runtime_external(this);
 
   ToDirectStringAssembler to_direct(state(), string);
   Node* const direct_string = to_direct.TryToDirect(&runtime_generic);
   Node* const offset = IntPtrAdd(int_index, to_direct.offset());
   Node* const instance_type = to_direct.instance_type();
 
@@ skipping 5338 matching lines @@
         formatted.c_str(), TENURED);
     CallRuntime(Runtime::kGlobalPrint, NoContextConstant(),
                 HeapConstant(string));
   }
   CallRuntime(Runtime::kDebugPrint, NoContextConstant(), tagged_value);
 #endif
 }
 
 }  // namespace internal
 }  // namespace v8