[Utils] Avoid repeated hash lookups (NFC) (llvm#130892)

Author: kazutakahirata

llvm/lib/Transforms/Utils/LoopUnrollAndJam.cpp

@@ -400,9 +400,10 @@ llvm::UnrollAndJamLoop(Loop *L, unsigned Count, unsigned TripCount,
       Last = New;
       for (ValueToValueMapTy::iterator VI = VMap.begin(), VE = VMap.end();
            VI != VE; ++VI) {
+        auto &LVM = LastValueMap[VI->first];
         PrevItValueMap[VI->second] =
-            const_cast<Value *>(It == 1 ? VI->first : LastValueMap[VI->first]);
-        LastValueMap[VI->first] = VI->second;
+            const_cast<Value *>(It == 1 ? VI->first : LVM);
+        LVM = VI->second;
       }
 
       NewBlocks.push_back(New);