Not all RegisterId values are registers, so Id is a more appropriate
name.

Use asMCReg() in some places that assumed it was a register.

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.

DenseSet, SmallPtrSet, SmallSet, SetVector, and StringSet recently
gained C++23-style insert_range.  This patch replaces:

  Dest.insert(Src.begin(), Src.end());

with:

  Dest.insert_range(Src);

This patch does not touch custom begin like succ_begin for now.

Update some other places to avoid implicit conversions this introduces,
but I probably missed some.

Many of these are indexing BitVectors or something where we can't
using MCRegister and need the register number.

Remove all instances of 'using namespace rdf' from files in CodeGen,
all implementation code is explicitly enclosed in 'namespace rdf'.

Recommit with the fix for incorrect `constexpr` with `hash` from a later
commit, and a fix for "std::swap(std::less<RegisterRef>...)" issue with
MSVC's `std::map`.