[ThinLTO] Fix Windows debug failure in new iterator

This fixes a debug assert on Windows from the new iterator
implementation added in r269059. The Windows std::vector iterator
operator== checks in debug mode that the containers being iterated over
are the same, which they may not be.

Fixed by checking that we are iterating over the same container before
comparing the container iterators.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@269232 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Bitcode/Writer/BitcodeWriter.cpp

@@ -333,21 +333,24 @@ public:
       // is empty. This will be handled specially in operator== as well.
       if (Writer.ModuleToSummariesForIndex &&
           !Writer.ModuleToSummariesForIndex->empty()) {
-        ModuleSummariesIter = Writer.ModuleToSummariesForIndex->begin();
         for (ModuleSummariesBack = Writer.ModuleToSummariesForIndex->begin();
              std::next(ModuleSummariesBack) !=
              Writer.ModuleToSummariesForIndex->end();
              ModuleSummariesBack++)
           ;
+        ModuleSummariesIter = !IsAtEnd
+                                  ? Writer.ModuleToSummariesForIndex->begin()
+                                  : ModuleSummariesBack;
         ModuleGVSummariesIter = !IsAtEnd ? ModuleSummariesIter->second.begin()
                                          : ModuleSummariesBack->second.end();
       } else if (!Writer.ModuleToSummariesForIndex &&
                  Writer.Index.begin() != Writer.Index.end()) {
-        IndexSummariesIter = Writer.Index.begin();
         for (IndexSummariesBack = Writer.Index.begin();
              std::next(IndexSummariesBack) != Writer.Index.end();
              IndexSummariesBack++)
           ;
+        IndexSummariesIter =
+            !IsAtEnd ? Writer.Index.begin() : IndexSummariesBack;
         IndexGVSummariesIter = !IsAtEnd ? IndexSummariesIter->second.begin()
                                         : IndexSummariesBack->second.end();
       }
@@ -398,6 +401,10 @@ public:
         // empty, they both are.
         if (Writer.ModuleToSummariesForIndex->empty())
           return true;
+        // Ensure the ModuleGVSummariesIter are iterating over the same
+        // container before checking them below.
+        if (ModuleSummariesIter != RHS.ModuleSummariesIter)
+          return false;
         return ModuleGVSummariesIter == RHS.ModuleGVSummariesIter;
       }
       // First ensure RHS also writing the full index, and that both are
@@ -409,6 +416,10 @@ public:
       // empty, they both are.
       if (Writer.Index.begin() == Writer.Index.end())
         return true;
+      // Ensure the IndexGVSummariesIter are iterating over the same
+      // container before checking them below.
+      if (IndexSummariesIter != RHS.IndexSummariesIter)
+        return false;
       return IndexGVSummariesIter == RHS.IndexGVSummariesIter;
     }
   };