[stdlib] Eliminate an overflow check in U[M]BP.distance

Author: lorentey

stdlib/public/core/UnsafeBufferPointer.swift.gyb

@@ -207,7 +207,14 @@ extension Unsafe${Mutable}BufferPointer: ${Mutable}Collection, RandomAccessColle
     // optimizer is not capable of creating partial specializations yet.
     // NOTE: Range checks are not performed here, because it is done later by
     // the subscript function.
-    return end - start
+    // NOTE: We allow the subtraction to silently overflow in release builds
+    // to eliminate a superflous check when `start` and `end` are both valid
+    // indices. (The operation can only overflow if `start` is negative, which
+    // implies it's an invalid index.) `Collection` does not specify what
+    // `distance` should return when given an invalid index pair.
+    let result = end.subtractingReportingOverflow(start)
+    _debugPrecondition(!result.overflow)
+    return result.partialValue
   }
 
   @inlinable // unsafe-performance