[CodeGen] Avoid repeated hash lookups (NFC) (#130889)

Author: kazutakahirata

llvm/lib/CodeGen/LiveRangeShrink.cpp

@@ -153,13 +153,13 @@ bool LiveRangeShrink::runOnMachineFunction(MachineFunction &MF) {
           continue;
         if (MO.isUse())
           UseMap[MO.getReg()] = std::make_pair(CurrentOrder, &MI);
-        else if (MO.isDead() && UseMap.count(MO.getReg()))
+        else if (MO.isDead()) {
           // Barrier is the last instruction where MO get used. MI should not
           // be moved above Barrier.
-          if (Barrier < UseMap[MO.getReg()].first) {
-            Barrier = UseMap[MO.getReg()].first;
-            BarrierMI = UseMap[MO.getReg()].second;
-          }
+          auto It = UseMap.find(MO.getReg());
+          if (It != UseMap.end() && Barrier < It->second.first)
+            std::tie(Barrier, BarrierMI) = It->second;
+        }
       }
 
       if (!MI.isSafeToMove(SawStore)) {