`size_hint` can be malformed from `rustc_middle::mir::traversal::Postorder`

[scottmcm]

Spotted with extra checking from 804cf26

size_hint (4, Some(2)) is malformed from iterator rustc_middle::mir::traversal::Postorder collecting into alloc::vec::Vec<rustc_middle::mir::BasicBlock>

query stack during panic:
#0 [analysis] running analysis passes on this crate
end of query stack
note: Clippy version: clippy 0.1.87 (7a632ba 2025-03-03)

[matthiaskrgr]

should this be a debug assertion? 🤔

[scottmcm]

@matthiaskrgr I definitely want to check in some form of checking this!

Probably as debug_assert, like you said. Not sure where would be best in the library; I'm tempted to try putting it in Iterator::collect, for example.