[CODEGEN] Bugfix in membar pass (#124)

Membar pass on top of master is buggy with asynchronous copy. For example, it doesn't wait for asynchronous copies to complete before recoalescing accumulator in GEMM, which leads to undefined behavior when the program doesn't enter the loop. This PR proposes

lib/ir/utils.cc

@@ -8,25 +8,39 @@
 namespace triton{
 namespace ir{
 
-std::vector<basic_block*> cfg::reverse_post_order(function* fn) {
+std::vector<basic_block*> cfg::post_order(function* fn) {
   std::stack<basic_block*> stack;
   std::set<basic_block*> visited;
   std::vector<basic_block*> result;
   // initialize stack
   for(ir::basic_block* block: fn->blocks())
-    if(block->get_predecessors().empty())
+    if(block->get_predecessors().empty()){
       stack.push(block);
+      visited.insert(block);
+    }
   // DFS
   while(!stack.empty()) {
     basic_block* current = stack.top();
-    stack.pop();
-    result.push_back(current);
-    visited.insert(current);
+    bool tail = true;
     for(basic_block* succ: current->get_successors())
-      if(visited.find(succ) == visited.end())
+      if(visited.find(succ) == visited.end()){
         stack.push(succ);
+        visited.insert(succ);
+        tail = false;
+        break;
+      }
+    if(tail){
+      stack.pop();
+      result.push_back(current);
+    }
   }
-  return std::move(result);
+  return result;
+}
+
+std::vector<basic_block*> cfg::reverse_post_order(function* fn) {
+  auto result = post_order(fn);
+  std::reverse(result.begin(), result.end());
+  return result;
 }
 
 void for_each_instruction(module &mod, const std::function<void (instruction *)> &do_work) {