[MC] Avoid repeated hash lookups (NFC) (#123698)

Author: kazutakahirata

llvm/lib/MC/WasmObjectWriter.cpp

@@ -746,10 +746,11 @@ static void addData(SmallVectorImpl<char> &DataBytes,
 uint32_t
 WasmObjectWriter::getRelocationIndexValue(const WasmRelocationEntry &RelEntry) {
   if (RelEntry.Type == wasm::R_WASM_TYPE_INDEX_LEB) {
-    if (!TypeIndices.count(RelEntry.Symbol))
+    auto It = TypeIndices.find(RelEntry.Symbol);
+    if (It == TypeIndices.end())
       report_fatal_error("symbol not found in type index space: " +
                          RelEntry.Symbol->getName());
-    return TypeIndices[RelEntry.Symbol];
+    return It->second;
   }
 
   return RelEntry.Symbol->getIndex();