Improve one of the comments in COWArrayOpts.

lib/SILOptimizer/LoopTransforms/COWArrayOpt.cpp

@@ -849,11 +849,16 @@ bool COWArrayOpt::hoistMakeMutable(ArraySemanticsCall MakeMutable,
 
   // Check whether we can hoist make_mutable based on the operations that are
   // in the loop.
-  // Note that in this case we don't verify that the array buffer is not aliased
-  // and therefore we must be conservative if the make_mutable is executed
-  // conditionally (i.e. doesn't dominate all exit blocks).
-  // The test SILOptimizer/cowarray_opt.sil: dont_hoist_if_executed_conditionally
-  // shows the problem.
+  //
+  // Hoisting make_mutable releases the original array storage. If an alias of
+  // that storage is accessed on any path reachable from the loop header that
+  // doesn't already pass through the make_mutable, then hoisting is
+  // illegal. hasLoopOnlyDestructorSafeArrayOperations checks that the array
+  // storage is not accessed within the loop. However, this does not include
+  // paths exiting the loop. Rather than analyzing code outside the loop, simply
+  // check that the original make_mutable dominates all exits. The test
+  // SILOptimizer/cowarray_opt.sil: dont_hoist_if_executed_conditionally shows
+  // the problem.
   if (hasLoopOnlyDestructorSafeArrayOperations() && dominatesExits) {
     // Done. We can hoist the make_mutable.
     // We still need the array uses later to check if we can add loads to