Rollup merge of #78857 - SkiFire13:bheap-opt, r=KodrAus
Rollup merge of #78857 - SkiFire13:bheap-opt, r=KodrAus

Improve BinaryHeap performance

By changing the condition in the loops from `child < end` to `child < end - 1` we're guaranteed that `right = child + 1 < end` and since finding the index of the biggest sibling can be done with an arithmetic operation we can remove a branch from the loop body. The case where there's no right child, i.e. `child == end - 1` is instead handled outside the loop, after it ends; note that if the loops ends early we can use `return` instead of `break` since the check `child == end - 1` will surely fail.

I've also removed a call to `<[T]>::swap` that was hiding a bound check that [wasn't being optimized by LLVM](https://godbolt.org/z/zrhdGM).

A quick benchmarks on my pc shows that the gains are pretty significant:

|name                 |before ns/iter  |after ns/iter  |diff ns/iter  |diff %    |speedup |
|---------------------|----------------|---------------|--------------|----------|--------|
|find_smallest_1000   | 352,565        | 260,098       |     -92,467  | -26.23%  | x 1.36 |
|from_vec             | 676,795        | 473,934       |    -202,861  | -29.97%  | x 1.43 |
|into_sorted_vec      | 469,511        | 304,275       |    -165,236  | -35.19%  | x 1.54 |
|pop                  | 483,198        | 373,778       |    -109,420  | -22.64%  | x 1.29 |

The other 2 benchmarks for `BinaryHeap` (`peek_mut_deref_mut` and `push`) weren't impacted and as such didn't show any significant change.

graydon/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust