[ModuleInliner] Remove InlineOrder::front (NFC)

[kazu]

Summary

InlineOrder::front is a remnant from the era when we had a nested
"while" loops in the module inliner, with the inner one grouping the
call sites with the same caller.

Now that we have a simple "while" loop draining the priority queue, we
can just use InlineOrder::pop.

llvm/include/llvm/Analysis/InlineOrder.h

public:
   virtual ~InlineOrder() = default;
 
   virtual size_t size() = 0;
 
   virtual void push(const T &Elt) = 0;
 
   virtual T pop() = 0;
 
-  virtual const_reference front() = 0;
-
   virtual void erase_if(function_ref<bool(T)> Pred) = 0;
 
   bool empty() { return !size(); }
 };
 
 std::unique_ptr<InlineOrder<std::pair<CallBase *, int>>>
 getInlineOrder(FunctionAnalysisManager &FAM, const InlineParams &Params);
 
 } // namespace llvm
 #endif // LLVM_ANALYSIS_INLINEORDER_H

llvm/lib/Analysis/InlineOrder.cpp

T pop() override {
     CallBase *CB = Heap.front();
     T Result = std::make_pair(CB, InlineHistoryMap[CB]);
     InlineHistoryMap.erase(CB);
     std::pop_heap(Heap.begin(), Heap.end(), isLess);
     Heap.pop_back();
     return Result;
   }
 
-  const_reference front() override {
-    assert(size() > 0);
-    adjust();
-
-    CallBase *CB = Heap.front();
-    return *InlineHistoryMap.find(CB);
-  }
-
   void erase_if(function_ref<bool(T)> Pred) override {
     auto PredWrapper = [=](CallBase *CB) -> bool {
       return Pred(std::make_pair(CB, 0));
     };
     llvm::erase_if(Heap, PredWrapper);
     std::make_heap(Heap.begin(), Heap.end(), isLess);
   }
 

llvm/lib/Transforms/IPO/ModuleInliner.cpp

PreservedAnalyses ModuleInlinerPass::run(Module &M,
   SmallVector<std::pair<Function *, int>, 16> InlineHistory;
 
   // Track the dead functions to delete once finished with inlining calls. We
   // defer deleting these to make it easier to handle the call graph updates.
   SmallVector<Function *, 4> DeadFunctions;
 
   // Loop forward over all of the calls.
   while (!Calls->empty()) {
-    Function &F = *Calls->front().first->getCaller();
-    (void)F;
+    auto P = Calls->pop();
+    CallBase *CB = P.first;
+    const int InlineHistoryID = P.second;
+    Function &F = *CB->getCaller();
+    Function &Callee = *CB->getCalledFunction();
 
     LLVM_DEBUG(dbgs() << "Inlining calls in: " << F.getName() << "\n"
                       << "    Function size: " << F.getInstructionCount()
                       << "\n");
+    (void)F;
 
     auto GetAssumptionCache = [&](Function &F) -> AssumptionCache & {
       return FAM.getResult<AssumptionAnalysis>(F);
     };
 
-    auto P = Calls->pop();
-    CallBase *CB = P.first;
-    const int InlineHistoryID = P.second;
-    Function &Callee = *CB->getCalledFunction();
-
     if (InlineHistoryID != -1 &&
         inlineHistoryIncludes(&Callee, InlineHistoryID, InlineHistory)) {
       setInlineRemark(*CB, "recursive");
       continue;
     }
 
     auto Advice = Advisor.getAdvice(*CB, /*OnlyMandatory*/ false);
     // Check whether we want to inline this callsite.