Clarify and partially enforce inst-order precondition on splicing (#6722)

Co-authored-by: Chandler Carruth <chandlerc@gmail.com>

toolchain/check/handle_loop_statement.cpp

@@ -160,6 +160,9 @@ auto HandleParseNode(Context& context, Parse::ForHeaderId node_id) -> bool {
   // Create the cursor variable.
   // TODO: Produce a custom diagnostic if the range operand can't be used as a
   // range.
+  // TODO: We need to allocate the `VarStorage` before building the operator.
+  // The current order risks violating the preconditions on `Initialize` and
+  // risks violating the topological ordering of insts.
   auto cursor_id =
       BuildUnaryOperator(context, node_id,
                          {.interface_name = CoreIdentifier::Iterate,