ICE: `polonius loan scopes differ from NLL borrow scopes` about rust HOT 6 OPEN

Amanda (and I, when I have a bit more time) will also look into this soon-ish to try and see where NLLs differ from the location-insensitive pass in this new situation.

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The problem is that NLL has an optimisation for computing loans out of scope that shortens the scope to skip the two final reborrows in the generated MIR body because borrowed_place.ignore_borrow() returns true, but Polonius doesn't so the scope for NLL is slightly shorter while the Polonius kill happens at the start of the cleanup/return blocks.

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Regression in nightly-2024-06-05
#125667 🤔 cc @oli-obk

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I can narrow that down to specifically:

outlives_constraints = Default::default();

Looking into a work-around!

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This only happens when the "remove constraints" trick of @oli-obk's is engaged because liveness values are merged into SCCs during construction of RegionInferenceContext, which happens after the constraints have been removed. This means that region liveness in Polonius still has the old view of the constraint graph via BorrowSet while NLL considers the rewritten graph.

Ironically this means that NLL is now slightly context-sensitive in the sense that parts of the constraint graph disappears between MIR statements.

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I don't think I can safely convert the work-around to Polonius since I'd have to either move the liveness logic to a later stage of computation (design change) or undo parts of the computation of the BorrowSet to exclude certain variables as holders of loans (scary, above my current power level).

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