Rollup merge of #121479 - lcnr:fix-generalize, r=compiler-errors

fix generalizer unsoundness

I ended up getting confused while trying to flip the variances when flipping the order. Should be all right now.

This is only exploitable when generalizing if the `ambient_variance` of the relation is `Contravariant`. This can currently only be the case in the NLL generalizer which only rarely generalizes, causing us to miss this regression. Very much an issue with #121462 however.
tree: ca6e420b05f1f4c3f5de4381cc76b8c98e9d8fd2
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