is_expr_identity_of_pat: simplify using (unordered_)over

[ada4a]

This makes the last arm significantly shorter, but marginally shortens the other ones as well

• I first removed the length comparisons from the match guards since they were already present in (unordered_)over, but then brought them back since they are quite a helpful "guard clause"
• It's a bit unfortunate that the Slice/Array arm can't use over as well, which creates some asymmetry, but changing the signature of over to accept arbitrary iterators felt like too much to me, especially because we'd lose the ability to compare the lengths of the two inputs (which could've been mitigated by ExactLenIterator::len, but that method is still unstable AFAIR)

One other option would be to only use unordered_over in the last arm, and not over, but I think over is not that bad..

changelog: none

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