I have read carefully about the ExprNode and StmtNode in include/tvm/ir.h and found that there are a few expressions and statements that are ignored in the C codegen during visiting. It is possible that these operations are always eliminated by previous passes, however, I think it is better to mark them as unreachable during visiting.
Expr(missing): FloorDiv, FloorMod, Reduce, Any
Stmt(missing): Provide, Free, Realize, Prefetch