[ty] Refactor if narrowing

[ericmarkmartin]

Summary

Refactor the semantic index builder’s If statement analysis. This change also lets us detect more unreachable code paths.

Test Plan

• Add new tests to unreachable.md
• Update existing tests

[carljm]

Looks like there are still some failing tests here? Let me know if any of these are things you'd like a second pair of eyes on how to address; otherwise I'll mark this as draft, go ahead and put it back up for review when CI is green.

[ericmarkmartin]

Looks like there are still some failing tests here? Let me know if any of these are things you'd like a second pair of eyes on how to address; otherwise I'll mark this as draft, go ahead and put it back up for review when CI is green.

Looks like we're green now, but mypy primer seems unhappy (sorry if that means I shouldn't have opened it, but I'm not sure how to proceed with primer failures)

[sharkdp]

Thank you for working on this!

Looks like we're green now, but mypy primer seems unhappy

The mypy_primer run was failing due to a network error. I re-ran it now and it fails with

fatal[panic] Panicked at crates/ty_python_semantic/src/symbol.rs:927:21 when checking `/tmp/mypy_primer/projects/trio/src/trio/_core/__init__.py`: `internal error: entered unreachable code: If we have at least one declaration, the scope-start should not be definitely visible`

This might be related to astral-sh/ty#365 (comment). I am planning to fix that bug as soon as #18041 is merged (which makes significant changes to the semantic index builder, and has a much higher impact). I suggest that we wait with this PR until these two things have been resolved.