[C++] Add versions of IsNull/IsValid that take an ArrowType tparam so implementation can be statically dispatched

[felipecrv]

Describe the enhancement requested

#34408 introduced more branches to the implementations of IsNull and IsValid testing the type to of the array to dispatch specific implementations for a few special types. Although these branches are easy on the branch predictor, it's good to offer users that know the type ahead of time a way to eliminate those branches completely from their loops.

Proposed implementation using if constexpr:

    template <typename ArrowType>
    bool IsNullFast(int64_t i) const {
      return !IsValidFast<ArrowType>(i);
    }

    template <typename ArrowType>
    bool IsValidFast(int64_t i) const {
      if constexpr (ArrowType::type_id == Type::NA) {
        return false;
      } else if constexpr (ArrowType::type_id == Type::SPARSE_UNION) {
        return !internal::IsNullSparseUnion(*this, i);
      } else if constexpr (ArrowType::type_id == Type::DENSE_UNION) {
        return !internal::IsNullDenseUnion(*this, i);
      } else if constexpr (ArrowType::type_id == Type::RUN_END_ENCODED) {
        return !internal::IsNullRunEndEncoded(*this, i);
      } else {
        if (buffers[0] != NULLPTR) {
          return bit_util::GetBit(buffers[0]->data(), i + offset);
        }
        return null_count.load() != length;
      }
  }

Component(s)

C++

[felipecrv]

take