Remove the last element before determining the insertion index (apple#173)

This could result in fewer comparisons.

Additionally, it ensures that the insertion index is valid for insertion. The `removeLast()` documentation states:
> Calling this method may invalidate all saved indices of this collection. Do not rely on a previously stored index value after altering a collection with any operation that can change its length.

Author: mdznr

Sources/Algorithms/MinMax.swift

@@ -28,11 +28,9 @@ extension Sequence {
     while let e = iterator.next() {
       // To be part of `result`, `e` must be strictly less than `result.last`.
       guard try areInIncreasingOrder(e, result.last!) else { continue }
+      result.removeLast()
       let insertionIndex =
         try result.partitioningIndex { try areInIncreasingOrder(e, $0) }
-      
-      assert(insertionIndex != result.endIndex)
-      result.removeLast()
       result.insert(e, at: insertionIndex)
     }