fix[venom]: clear out_vars during calculation (#4129)

Clear `out_vars` set at start of calculations

When computing `out_vars` for each basic block, we perform set unions
of variables expected by all successor basic blocks of the current block.
This commit clears the `out_vars` set at the beginning of the calculations.

---------

Co-authored-by: Charles Cooper <cooper.charles.m@gmail.com>

vyper/venom/analysis/liveness.py

@@ -54,7 +54,8 @@ def _calculate_out_vars(self, bb: IRBasicBlock) -> bool:
         Compute out_vars of basic block.
         Returns True if out_vars changed
         """
-        out_vars = bb.out_vars.copy()
+        out_vars = bb.out_vars
+        bb.out_vars = OrderedSet()
         for out_bb in bb.cfg_out:
             target_vars = self.input_vars_from(bb, out_bb)
             bb.out_vars = bb.out_vars.union(target_vars)