[IR] Dereference nullptr

[sjwsl]

Using nullptr as a parameter here will cause the dereference of nullptr later, which is an undefined behavior.

taichi/taichi/ir/frontend_ir.cpp

Lines 681 to 685 in 0888810

	ctx->push_back<SNodeOpStmt>(SNodeOpType::is_active, snode, ptr, nullptr);
	} else if (op_type == SNodeOpType::length) {
	ctx->push_back<SNodeOpStmt>(SNodeOpType::length, snode, ptr, nullptr);
	} else if (op_type == SNodeOpType::get_addr) {
	ctx->push_back<SNodeOpStmt>(SNodeOpType::get_addr, snode, ptr, nullptr);

This is one of the dereferences, which is the root cause of #3323.

taichi/taichi/transforms/make_thread_local.cpp

Line 80 in 5a18210

if (auto atomic = op->cast<AtomicOpStmt>()) {

Just to fix this one is easy, but there may (very likely) be similar errors elsewhere not found. I think either we need something like NullStmt to completely avoid nullptr for Stmt*, or check it before each dereference (which is much more verbose).