Remove checks in UR[M]BP.Iterator.next() (#62965)

Swift tends to emit unnecessary checks and traps when iterating unsafe
raw buffer pointers. These traps are confirming that the position
pointer isn't nil, but this check is redundant with the bounds check
that is already present. We can safely remove it.

Author: Lukasa

stdlib/public/core/UnsafeRawBufferPointer.swift.gyb

@@ -148,8 +148,21 @@ extension UnsafeRawBufferPointer.Iterator: IteratorProtocol, Sequence {
   public mutating func next() -> UInt8? {
     if _position == _end { return nil }
 
-    let result = _position!.load(as: UInt8.self)
-    _position! += 1
+    // We can do an unchecked unwrap here by borrowing invariants from the pointer.
+    // For a validly constructed buffer pointer, the only way _position can be nil is
+    // if _end is also nil. We checked that case above. Thus, we can safely do an
+    // unchecked unwrap here.
+    //
+    // Additionally, validly constructed buffer pointers also have an _end that is
+    // strictly greater than or equal to _position, and so we do not need to do checked
+    // arithmetic here as we cannot possibly overflow.
+    //
+    // We check these invariants in debug builds to defend against invalidly constructed
+    // pointers.
+    _debugPrecondition(_position! < _end!)
+    let position = _position._unsafelyUnwrappedUnchecked
+    let result = position.load(as: UInt8.self)
+    _position = position + 1
     return result
   }
 }