[red-knot] add while-loop to benchmark (#12464)

So we can get some signal from the benchmark result on
https://github.com/astral-sh/ruff/pull/12413

crates/ruff_benchmark/benches/red_knot.rs

@@ -36,6 +36,8 @@ class Bar:
             return 48472783
         if arg < 10:
             return 20
+        while arg < 50:
+            arg += 1
         return 36673
 "#;