[CodeGen] Get rid of incorrect `std` template specializations (#160804)

This patch renames comparators
- from `std::equal_to<llvm::rdf::RegisterRef>` to
`llvm::rdf::RegisterRefEqualTo`, and
- from `std::less<llvm::rdf::RegisterRef>` to
`llvm::rdf::RegisterRefLess`.

The original specializations don't satisfy the requirements for the
original `std` templates by being stateful and
non-default-constructible, so they make the program have UB due to C++17
[namespace.std]/2, C++20/23 [namespace.std]/5.

> A program may explicitly instantiate a class template defined in the
standard library only if the declaration
> - depends on the name of at least one program-defined type, and
> - the instantiation meets the standard library requirements for the
original template.