[ADT] Use try_emplace in SCCIterator.h (NFC) (llvm#164187)

This patch replaces an unidiomatic operator[] usage with try_emplace
to ensure in-place NodeInfo construction and correct Group field
initialization.

This patch also clarifies the comment.  In particular, "insertion
operation" by itself isn't clear whether it's referring to insert() or
all of insertion operations like insert(), insert_or_assign(),
try_emplace(), etc.

Author: kazutakahirata

llvm/include/llvm/ADT/SCCIterator.h

@@ -313,10 +313,10 @@ scc_member_iterator<GraphT, GT>::scc_member_iterator(
   // Initialize auxilary node information.
   NodeInfoMap.clear();
   for (auto *Node : InputNodes) {
-    // This is specifically used to construct a `NodeInfo` object in place. An
-    // insert operation will involve a copy construction which invalidate the
-    // initial value of the `Group` field which should be `this`.
-    (void)NodeInfoMap[Node].Group;
+    // Construct a `NodeInfo` object in place.  `insert()` would involve a copy
+    // construction, invalidating the initial value of the `Group` field, which
+    // should be `this`.
+    NodeInfoMap.try_emplace(Node);
   }
 
   // Sort edges by weights.