Draft: CPR analysis: Don't lose shared values (#19326)

This patch tries hard not to give value bindings the CPR property in multi-shot contexts and regresses terribly while doing so.

Lacks documentation, but the gist is to track which binders are defined at the current multi-shot level (e.g. there is no multi-shot lambda between the definition and the current analysis site) in ae_same_lvl :: IdSet and refrain from unleashing the CPR signature of arity 0 things if the thing is defined outside the current multi-shot level. ae_same_lvl is extended at each binding site and cleared when analysis passes a multi-shot lambda.

From what I can see, the analysis now regresses in quite a few places, notably product [1..n] won't have the CPR property because the 1 case is floated out. Let's see what else CI has to say though.