[Utils] Avoid repeated hash lookups (NFC)

Author: kazutakahirata

llvm/lib/Transforms/Utils/LoopUnrollAndJam.cpp

@@ -395,8 +395,9 @@ llvm::UnrollAndJamLoop(Loop *L, unsigned Count, unsigned TripCount,
       }
 
       // Update our running maps of newest clones
-      PrevItValueMap[New] = (It == 1 ? *BB : LastValueMap[*BB]);
-      LastValueMap[*BB] = New;
+      auto &Last = LastValueMap[*BB];
+      PrevItValueMap[New] = (It == 1 ? *BB : Last);
+      Last = New;
       for (ValueToValueMapTy::iterator VI = VMap.begin(), VE = VMap.end();
            VI != VE; ++VI) {
         PrevItValueMap[VI->second] =