[StructurizeCFG][DebugInfo] Avoid use-after-free

[dstuttard]

This fixes the issue I was observing. Thanks.

[jmmartinez]

Rebase

llvm/lib/Transforms/Scalar/StructurizeCFG.cpp

 /// Create a new flow node and update dominator tree and region info
 BasicBlock *StructurizeCFG::getNextFlow(BasicBlock *Dominator) {
   LLVMContext &Context = Func->getContext();
   BasicBlock *Insert = Order.empty() ? ParentRegion->getExit() :
                        Order.back()->getEntry();
   BasicBlock *Flow = BasicBlock::Create(Context, FlowBlockName,
                                         Func, Insert);
   FlowSet.insert(Flow);
-  TermDL[Flow] = TermDL[Dominator];
+
+  // use a temporary variable to avoid a use-after-free if the map's storage is
+  // reallocated
+  DebugLoc DL = TermDL[Dominator];
+  TermDL[Flow] = std::move(DL);
+
   DT->addNewBlock(Flow, Dominator);
   ParentRegion->getRegionInfo()->setRegionFor(Flow, ParentRegion);
   return Flow;
 }
 
 /// Create a new or reuse the previous node as flow node
 BasicBlock *StructurizeCFG::needPrefix(bool NeedEmpty) {
   BasicBlock *Entry = PrevNode->getEntry();