[NFC] Add a utility to DiverseStack to simplify stable iteration

Author: rjmccall

include/swift/Basic/DiverseStack.h

@@ -294,6 +294,15 @@ template <class T> class DiverseStackImpl : private DiverseStackBase {
     return stable_iterator(End - it.Ptr);
   } 
 
+  T &findAndAdvance(stable_iterator &i) {
+    auto unstable_i = find(i);
+    assert(unstable_i != end());
+    T &value = *unstable_i;
+    ++unstable_i;
+    i = stabilize(unstable_i);
+    return value;
+  }
+
   class const_iterator {
     const char *Ptr;
     friend class DiverseStackImpl;

lib/SILGen/Cleanup.cpp

@@ -91,20 +91,21 @@ void CleanupManager::emitCleanups(CleanupsDepth depth, CleanupLocation loc,
   auto topOfStack = cur;
 #endif
   while (cur != depth) {
+    // Advance the iterator.
+    auto cleanupHandle = cur;
+    auto iter = stack.find(cleanupHandle);
+    Cleanup &stackCleanup = *iter;
+    cur = stack.stabilize(++iter);
+
     // Copy the cleanup off the stack if it needs to be emitted.
     // This is necessary both because we might need to pop the cleanup and
     // because the cleanup might push other cleanups that will invalidate
     // references onto the stack.
-    auto iter = stack.find(cur);
-    Cleanup &stackCleanup = *iter;
     Optional<CleanupBuffer> copiedCleanup;
     if (stackCleanup.isActive() && SGF.B.hasValidInsertionPoint()) {
       copiedCleanup.emplace(stackCleanup);
     }
 
-    // Advance the iterator.
-    cur = stack.stabilize(++iter);
-
     // Pop now if that was requested.
     if (popCleanups) {
       stack.pop();

lib/SILGen/Cleanup.h

@@ -165,6 +165,10 @@ class LLVM_LIBRARY_VISIBILITY CleanupManager {
     return *stack.find(iter);
   }
 
+  Cleanup &findAndAdvance(CleanupsDepth &iter) {
+    return stack.findAndAdvance(iter);
+  }
+
   /// \brief Emit a branch to the given jump destination,
   /// threading out through any cleanups we need to run. This does not pop the
   /// cleanup stack.