Fix #22604: Propagate info about terminating branches

[SirOlaf]

Close #22604

Propagates a new nfEndBlock flag for branches that terminate the block, improves type inference and unreachable code detection.
I don't know what sempass2 is for, so if that needs to handle this situation too please provide feedback.

Thinks the echo for "hey iterator" in #20362 is unreachable even though it prints, so either that case needs to be made consistent or the propagation needs to somehow work around this behavior.

Alternative to this pr would be making endsInNoReturn check the appropriate node types.

[Araq]

Alternative to this pr would be making endsInNoReturn check the appropriate node types.

That's what you need to do. Node flags are unreliable and the wrong idea.

[SirOlaf]

Doing that seemed more expensive to me because you gotta traverse the tree each time and keep track of the branches. Other problem is that the unreachable code check does not actually rely on it either (less problematic).

[Araq]

Actually, using some special internal type for this seems to be both easier and faster.

[SirOlaf]

Yes like that never type brought up by @metagn, effectively the same as this pr but instead of a weird flag it's an actual part of the type system

[Araq]

Not quite, it would be an internal type for now that should not be exposed.