kernel/rtlil: Extract some helpers for checking memory cell types.

There will soon be more (versioned) memory cells, so handle passes that
only care if a cell is memory-related by a simple helper call instead of
a hardcoded list.

backends/btor/btor.cc

@@ -708,7 +708,7 @@ struct BtorWorker
 			goto okay;
 		}
 
-		if (cell->type.in(ID($mem), ID($memrd), ID($memwr), ID($meminit)))
+		if (cell->is_mem_cell())
 		{
 			Mem *mem = mem_cells[cell];