Revert recent dead-code elimination improvement in constant prop.

lib/SILOptimizer/Utils/ConstantFolding.cpp

@@ -1818,6 +1818,15 @@ ConstantFolder::processWorkList() {
     }
 
     // Go through all users of the constant and try to fold them.
+    //
+    // FIXME: remove this temporary deleter. It is dangerous because any use of
+    // the original deleter will invalidate its iterators. It is currently used
+    // to work around bugs that are exposed in the -Onone stdlib build when the
+    // same deleter is used for both the dead code elimination above and the
+    // dead use elimination below.
+    auto tempCallbacks = deleter.getCallbacks();
+    InstructionDeleter tempDeleter(std::move(tempCallbacks));
+
     for (auto Result : I->getResults()) {
       for (auto *Use : Result->getUses()) {
         SILInstruction *User = Use->getUser();
@@ -1841,7 +1850,7 @@ ConstantFolder::processWorkList() {
         // this as part of the constant folding logic, because there is no value
         // they can produce (other than empty tuple, which is wasteful).
         if (isa<CondFailInst>(User))
-          deleter.trackIfDead(User);
+          tempDeleter.trackIfDead(User);
 
         // See if we have an instruction that is read none and has a stateless
         // inverse. If we do, add it to the worklist so we can check its users
@@ -1947,15 +1956,15 @@ ConstantFolder::processWorkList() {
           // it, we exit the worklist as expected.
           SILValue r = User->getResult(Index);
           if (r->use_empty()) {
-            deleter.trackIfDead(User);
+            tempDeleter.trackIfDead(User);
             continue;
           }
 
           // Otherwise, do the RAUW.
           User->getResult(Index)->replaceAllUsesWith(C);
           // Record the user if it is dead to perform the necessary cleanups
           // later.
-          deleter.trackIfDead(User);
+          tempDeleter.trackIfDead(User);
 
           // The new constant could be further folded now, add it to the
           // worklist.
@@ -1967,7 +1976,7 @@ ConstantFolder::processWorkList() {
 
     // Eagerly DCE. We do this after visiting all users to ensure we don't
     // invalidate the uses iterator.
-    deleter.cleanupDeadInstructions();
+    tempDeleter.cleanupDeadInstructions();
   }
 
   // TODO: refactor this code outside of the method. Passes should not merge