JIT: Consider making handles their own node type

[jakobbotsch]

The JIT has had a number of bugs related to the fact that handles are special and cannot participate in various opts, even though we represent them as GT_CNS_INT. #104634 is one latest example.
I think we should consider introducing a dedicated node type for the (opaque) handles to make these bugs less likely to be introduced.

cc @dotnet/jit-contrib

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[SingleAccretion]

Consideration: under Jit (!opts.compReloc) it is ok to dissolve handles (they have their final values). It is only under AOT that handles are special.

[jakobbotsch]

@SingleAccretion Yeah, that's what I was alluding to by the "(opaque)". Though it would be nice to remove the handle flags from GT_CNS_INT as well, but we use those for a few things (mostly to annotate disassembly, I think), so we probably wouldn't be able to do that if we left the runtime JIT handles as GT_CNS_INT.