Update a FIXME in memory.rs

In  #51833, I improved the performance of `copy_undef_mask()`. As such, the old FIXME wasn't appropriate anymore. The main remaining thing left to do is to implement a fast path for non-overlapping copies (per @oli-obk).

src/librustc_mir/interpret/memory.rs

@@ -971,7 +971,7 @@ impl<'a, 'mir, 'tcx, M: Machine<'a, 'mir, 'tcx>> Memory<'a, 'mir, 'tcx, M> {
 
 /// Undefined bytes
 impl<'a, 'mir, 'tcx, M: Machine<'a, 'mir, 'tcx>> Memory<'a, 'mir, 'tcx, M> {
-    // FIXME(solson): This is a very naive, slow version.
+    // FIXME: Add a fast version for the common, nonoverlapping case
     fn copy_undef_mask(
         &mut self,
         src: Pointer,