[yashaggarwal85d]

When an array may have been partially moved out by an array pattern, drop elaboration splits it into
kept indices and dropped subslices, but then built the drop ladder over those pieces in reverse. So
let a = [D(1), D(2)]; if false { let [_x, _] = a; } dropped 2 before 1. The reversal came in with
#109008; before that the ladder went first to last, which is what the Reference says.

This removes the .rev() and adds a run-pass test. Two existing drop-order tests
(if-let-guards.rs, or-pattern-drop-order.rs) were asserting the reversed order, so I fixed their expectations.

Fixes #164809

AI was used to write tests for this. I reviewed the changes myself. Thanks

[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

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[petrochenkov]

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