Load on vec does not allow pointer to non-const data.

The `load` member function on `sycl::vec` takes a `multi_ptr` which seems to written that way to allow pointers with and without `const` data. However, because the function is dependent, the conversion from `multi_ptr` to `multi_ptr` cannot be resolved. See https://godbolt.org/z/dTdc6K7a9.

Possible solution: Make two overloads of `load` - one with `multi_ptr, ...>` and one with `multi_ptr, ...>`. See https://godbolt.org/z/xs175dc7v.

Inspired by https://github.com/KhronosGroup/SYCL-CTS/pull/870.

Start at the sycl::vec::load member function and reproduce the failure using the Godbolt example linked in the issue. Compare the proposed overload approach with the second Godbolt example, and consider the behavior demonstrated by SYCL-CTS pull request 870; done means loading from a non-const multi_ptr is accepted.

Written by the indexing model from the issue text.