PLR1714: Repeated equality comparison to the same literal value shouldn't convert to a set

[injust]

Summary

Observed this in and split it out of #21690.

def test(foo: str) -> None:
    if foo == "bar" or foo == "bar":
        pass

https://play.ruff.rs/40871aec-3cf3-48fd-b42f-76f1b8c4118f

PLR1714 changes this to:

def test(foo: str) -> None:
    if foo in {"bar", "bar"}:
        pass

If the values being compared are literals, the equality comparisons should be merged instead of converting to a set.

[MichaReiser]

I'm leaning towards not flagging PLR1714 if all members are equal because fixing it to a single equal isn't what the rule is testing for. Detecting that the second equal is unnecessary is something that falls into a generic unreachable rule