[msl] clean up emitted expressions at the end of a block

src/back/msl/writer.rs

@@ -969,6 +969,16 @@ impl<W: Write> Writer<W> {
                 }
             }
         }
+
+        // un-emit expressions
+        //TODO: take care of loop/continuing?
+        for statement in statements {
+            if let crate::Statement::Emit(ref range) = *statement {
+                for handle in range.clone() {
+                    self.named_expressions.remove(handle.index());
+                }
+            }
+        }
         Ok(())
     }