Incorrect detection of unused variables in generators

[PabloRuizCuevas]

Describe the Bug

Some generators with send incorrectly report that sent variables are unused

from typing import Generator, cast

def test() -> Generator[float, float]:
    """absolute difference first last step"""
    new = yield None
    while True:
        new = yield new - 1  # `Variable `new` is unusedPyrefly(unused-variable)`

This can't be seen in the sandbox, but in vs code

Sandbox Link

No response

(Only applicable for extension issues) IDE Information

No response

[stroxler]

cc @kinto0 - I think we probably aren't accounting for Phi properly in our unused variable logic. I probably should have noticed this, a basic AST traversal just looking for reads is never going to handle loops right because of the loopback dependency