Replace FunctionContext::IsArgConstant(int i) with constant during codegen

TimestampVal TimestampFunctions::ToTimestamp(FunctionContext* context,
    const StringVal& date, const StringVal& fmt) {
  if (fmt.is_null || fmt.len == 0) {
    TimestampFunctions::ReportBadFormat(context, fmt, false);
    return TimestampVal::null();
  }
  if (date.is_null || date.len == 0) return TimestampVal::null();
  void* state = context->GetFunctionState(FunctionContext::THREAD_LOCAL);
  DateTimeFormatContext* dt_ctx = reinterpret_cast<DateTimeFormatContext*>(state);
  if (!context->IsArgConstant(1)) {
     dt_ctx->Reset(reinterpret_cast<const char*>(fmt.ptr), fmt.len);
     if (!TimestampParser::ParseFormatTokens(dt_ctx)) {
       ReportBadFormat(context, fmt, false);
       return TimestampVal::null();
     }
  }
  const TimestampValue& tv = TimestampValue::Parse(
      reinterpret_cast<const char*>(date.ptr), date.len, *dt_ctx);
  TimestampVal tv_val;
  tv.ToTimestampVal(&tv_val);
  return tv_val;
}

StringVal StringFunctions::BTrimString(FunctionContext* ctx,
    const StringVal& str, const StringVal& chars_to_trim) {
  if (str.is_null) return StringVal::null();
  bitset<256>* unique_chars = reinterpret_cast<bitset<256>*>(
      ctx->GetFunctionState(FunctionContext::THREAD_LOCAL));
  // When 'chars_to_trim' is unique for each element (e.g. when 'chars_to_trim'
  // is each element of a table column), we need to prepare a bitset of unique
  // characters here instead of using the bitset from function context.
  if (!ctx->IsArgConstant(1)) {
    unique_chars->reset();
    DCHECK(chars_to_trim.len != 0 || chars_to_trim.is_null);
    for (int32_t i = 0; i < chars_to_trim.len; ++i) {
      unique_chars->set(static_cast<int>(chars_to_trim.ptr[i]), true);
    }
  }
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