[regexp] Address various minor nits

src/code-stub-assembler.cc

Index: src/code-stub-assembler.cc
diff --git a/src/code-stub-assembler.cc b/src/code-stub-assembler.cc
index 03c80d2ed424cf755e3da7f93648f23b53ba36eb..fb0ffc9b39456e63a5515a926c242e6821373d32 100644
--- a/src/code-stub-assembler.cc
+++ b/src/code-stub-assembler.cc
@@ -3154,10 +3154,12 @@ Node* CodeStubAssembler::IsJSRegExp(Node* object) {
 
 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);