Auto merge of #43713 - arielb1:legacy-dataflow, r=eddyb

rustc::middle::dataflow - visit the CFG in RPO

We used to propagate bits in node-id order, which sometimes caused an
excessive number of iterations, especially when macros were present. As
everyone knows, visiting the CFG in RPO bounds the number of iterators
by 1 plus the depth of the most deeply nested loop (times the height of
the lattice, which is 1).

I have no idea how this affects borrowck perf in the non-worst-case, so it's probably a good idea to not roll this up so we can see the effects.

Fixes #43704.

r? @eddyb

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