[Hexagon] Avoid repeated hash lookups (NFC) (#130545)

Author: kazutakahirata

llvm/lib/Target/Hexagon/HexagonISelDAGToDAG.cpp

@@ -2022,8 +2022,9 @@ static bool isTargetConstant(const SDValue &V) {
 }
 
 unsigned HexagonDAGToDAGISel::getUsesInFunction(const Value *V) {
-  if (GAUsesInFunction.count(V))
-    return GAUsesInFunction[V];
+  auto [It, Inserted] = GAUsesInFunction.try_emplace(V);
+  if (!Inserted)
+    return It->second;
 
   unsigned Result = 0;
   const Function &CurF = CurDAG->getMachineFunction().getFunction();
@@ -2033,7 +2034,7 @@ unsigned HexagonDAGToDAGISel::getUsesInFunction(const Value *V) {
       ++Result;
   }
 
-  GAUsesInFunction[V] = Result;
+  It->second = Result;
 
   return Result;
 }