[dfsan] Rename CachedCombinedShadow to be CachedShadow

[stephan.yichao.zhao]

Summary

At D92261, this type will be used to cache both combined shadow and
converted shadow values

[morehouse]

llvm/lib/Transforms/Instrumentation/DataFlowSanitizer.cpp

431

Nit: This comment could fit on the same line as Block.

[stephan.yichao.zhao]

updated comments

llvm/lib/Transforms/Instrumentation/DataFlowSanitizer.cpp

struct DFSanFunction {
   AllocaInst *LabelReturnAlloca = nullptr;
   DenseMap<Value *, Value *> ValShadowMap;
   DenseMap<AllocaInst *, AllocaInst *> AllocaShadowMap;
   std::vector<std::pair<PHINode *, PHINode *>> PHIFixups;
   DenseSet<Instruction *> SkipInsts;
   std::vector<Value *> NonZeroChecks;
   bool AvoidNewBlocks;
 
-  struct CachedCombinedShadow {
-    BasicBlock *Block;
+  struct CachedShadow {
+    BasicBlock *Block; // The block where Shadow is defined.
     Value *Shadow;
   };
-  DenseMap<std::pair<Value *, Value *>, CachedCombinedShadow>
-      CachedCombinedShadows;
+  /// Maps a value to its latest shadow value in terms of domination tree.
+  DenseMap<std::pair<Value *, Value *>, CachedShadow> CachedShadows;
   DenseMap<Value *, std::set<Value *>> ShadowElements;
 
   DFSanFunction(DataFlowSanitizer &DFS, Function *F, bool IsNativeABI)
       : DFS(DFS), F(F), IA(DFS.getInstrumentedABI()), IsNativeABI(IsNativeABI) {
     DT.recalculate(*F);
     // FIXME: Need to track down the register allocator issue which causes poor
     // performance in pathological cases with large numbers of basic blocks.
     AvoidNewBlocks = F->size() > 1000;
Value *DFSanFunction::combineShadows(Value *V1, Value *V2, Instruction *Pos) {
   } else if (V2Elems != ShadowElements.end()) {
     if (V2Elems->second.count(V1))
       return V2;
   }
 
   auto Key = std::make_pair(V1, V2);
   if (V1 > V2)
     std::swap(Key.first, Key.second);
-  CachedCombinedShadow &CCS = CachedCombinedShadows[Key];
+  CachedShadow &CCS = CachedShadows[Key];
   if (CCS.Block && DT.dominates(CCS.Block, Pos->getParent()))
     return CCS.Shadow;
 
   IRBuilder<> IRB(Pos);
   if (ClFast16Labels) {
     CCS.Block = Pos->getParent();
     CCS.Shadow = IRB.CreateOr(V1, V2);
   } else if (AvoidNewBlocks) {