Fix --strict-equality for iteratively visited code

[tyralla]

Fixes #19328

The logic is very similar to what we did to report different revealed types that were discovered in multiple iteration steps in one line. I think this fix is the last one needed before I can implement #19256.

[tyralla]

The primer results are a little mysterious. I could simplify to:

for y in [1.0]:
    if y is not None or y != "None":  # E: Non-overlapping equality check (left operand type: "float", right operand type: "Literal['None']")
        ...

This obvious error is only reported with my change, but I do not know why current master misses it and therefore have no idea what in this PR contributes to detecting it.

For whatever reason this happens, I will add a corresponding test case.

[tyralla]

Ah, I got it. It is because optuna enables --strict-equality but not unreachable. y is not None is always true, so that the y != "None" mismatch is not reported.

I could invest some time so that

for y in [1.0]:
    if y is not None or y != "None":
        ...

and

y = 1.0
if y is not None or y != "None":
    ...

would be handled identically again (no --strict-equality warnings). However, this is a good example that failing to raise warnings in unreachable code can be confusing, and there is already a PR that tries to improve the situation (#18707). Hence, maybe no further action is required here.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

core (https://github.com/home-assistant/core)
+ homeassistant/components/homekit/__init__.py:982: error: Unused "type: ignore" comment  [unused-ignore]

urllib3 (https://github.com/urllib3/urllib3)
+ src/urllib3/connectionpool.py:1056: error: Unused "type: ignore" comment  [unused-ignore]

werkzeug (https://github.com/pallets/werkzeug)
+ src/werkzeug/datastructures/accept.py:259: error: Unused "type: ignore" comment  [unused-ignore]
+ src/werkzeug/datastructures/accept.py:264: error: Unused "type: ignore" comment  [unused-ignore]