bugfix: only the first occurance of a label counts

Because `insert` updates on repeated insertions, this is probably the easiest way.

test-files/snippets/analysis/labels.txt

@@ -2,4 +2,5 @@ Lbl RE
 Menu("MY TERRIBLE GAME","PLAY",PL,"EXIT",0,"PLEASE LEAVE",0,"RESTART",RE
 Lbl PL
 Goto 0
-Lbl 0
+Lbl 0
+Lbl PL

ti-basic-optimizer/src/analyze/control_flow/labels.rs

@@ -11,7 +11,7 @@ impl Program {
     pub fn label_declarations(&self) -> BTreeMap<LabelName, usize> {
         let mut declarations = BTreeMap::new();
 
-        for (idx, line) in self.lines.iter().enumerate() {
+        for (idx, line) in self.lines.iter().enumerate().rev() {
             if let Command::ControlFlow(ControlFlow::Lbl(name)) = line {
                 declarations.insert(*name, idx);
             }