CFG: revert change for single-stmt block

Author: sipma

chb/app/Cfg.py

@@ -477,6 +477,8 @@ def stmt_ast(
         fn = astfn.function
 
         def mk_block(stmts: List[AST.ASTStmt]) -> AST.ASTStmt:
+            if len(stmts) == 1 and not stmts[0].is_ast_instruction_sequence:
+                return stmts[0]
             return astree.mk_block(stmts)
 
         gotolabels: Set[str] = set() # this is both used and mutated by run_with_gotolabels()