[TLX][AMD] isFedByTDM: switch to pop_back_val worklist idiom

Index-iterate-while-inserting on a SetVector works (Value::getUsers
snapshots Operation-level iterators independent of SetVector storage),
but it's a non-obvious pattern. Switch to the more idiomatic
`pop_back_val` form which makes the value lifetime explicit (no
reference into possibly-reallocated storage). DFS instead of BFS
order is benign here — the function is a short-circuiting any-exists
query over a tree-shaped memdesc graph.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: Hardcode84

third_party/tlx/dialect/lib/Transforms/InsertRequireLayout.cpp

@@ -280,8 +280,9 @@ static Value findMemDescRoot(Value memdesc) {
 static bool isFedByTDM(Value memdesc) {
   llvm::SetVector<Value> worklist;
   worklist.insert(findMemDescRoot(memdesc));
-  for (size_t i = 0; i < worklist.size(); ++i) {
-    for (Operation *u : worklist[i].getUsers()) {
+  while (!worklist.empty()) {
+    Value v = worklist.pop_back_val();
+    for (Operation *u : v.getUsers()) {
       if (isa<amdgpu::AsyncTDMCopyGlobalToLocalOp>(u))
         return true;
       if (isa<ttg::MemDescIndexOp, ttg::MemDescReinterpretOp>(u))