Don't modify the DenseMap being iterated over from within the loop

[sanjoy]

Summary

Inserting elements into a DenseMap invalidated iterators pointing into the DenseMap instance.

lib/CodeGen/CodeGenPrepare.cpp

static void computeBaseDerivedRelocateMap(
   for (auto &Item : RelocateIdxMap) {
     std::pair<unsigned, unsigned> Key = Item.first;
     if (Key.first == Key.second)
       // Base relocation: nothing to insert
       continue;
 
     IntrinsicInst *I = Item.second;
     auto BaseKey = std::make_pair(Key.first, Key.first);
-    IntrinsicInst *Base = RelocateIdxMap[BaseKey];
-    if (!Base)
+
+    // We're iterating over RelocateIdxMap so we cannot modify it.
+    auto MaybeBase = RelocateIdxMap.find(BaseKey);
+    if (MaybeBase == RelocateIdxMap.end())
       // TODO: We might want to insert a new base object relocate and gep off
       // that, if there are enough derived object relocates.
       continue;
-    RelocateInstMap[Base].push_back(I);
+
+    RelocateInstMap[MaybeBase->second].push_back(I);
   }
 }
 
 // Accepts a GEP and extracts the operands into a vector provided they're all
 // small integer constants
 static bool getGEPSmallConstantIntOffsetV(GetElementPtrInst *GEP,
                                           SmallVectorImpl<Value *> &OffsetV) {
   for (unsigned i = 1; i < GEP->getNumOperands(); i++) {