Rollup merge of #90099 - SkiFire13:fix-vec-swap-remove, r=dtolnay

Fix MIRI UB in `Vec::swap_remove`

Fixes #90055

I find it weird that `Vec::swap_remove` read the last element to the stack just to immediately put it back in the `Vec` in place of the one at index `index`. It seems much more natural to me to just read the element at position `index` and then move the last element in its place. I guess this might also slightly improve codegen.