[MC] Avoid repeated hash lookups (NFC) (llvm#110791)

Author: kazutakahirata

llvm/lib/MC/XCOFFObjectWriter.cpp

@@ -666,8 +666,9 @@ void XCOFFObjectWriter::recordRelocation(MCAssembler &Asm,
     // If we could not find the symbol directly in SymbolIndexMap, this symbol
     // could either be a temporary symbol or an undefined symbol. In this case,
     // we would need to have the relocation reference its csect instead.
-    return SymbolIndexMap.contains(Sym)
-               ? SymbolIndexMap[Sym]
+    auto It = SymbolIndexMap.find(Sym);
+    return It != SymbolIndexMap.end()
+               ? It->second
                : SymbolIndexMap[ContainingCsect->getQualNameSymbol()];
   };