[Hexagon] Avoid repeated map lookups (NFC) (#127447)

Author: kazutakahirata

llvm/lib/Target/Hexagon/HexagonCommonGEP.cpp

@@ -604,8 +604,10 @@ void HexagonCommonGEP::common() {
       uint32_t NF = N->Flags;
       // If N is used, append all original values of N to the list of
       // original values of Min.
-      if (NF & GepNode::Used)
-        MinUs.insert(Uses[N].begin(), Uses[N].end());
+      if (NF & GepNode::Used) {
+        auto &U = Uses[N];
+        MinUs.insert(U.begin(), U.end());
+      }
       Flags |= NF;
     }
     if (MinUs.empty())