add unique symbols check to venom pipeline

when `-Onone` is specified along with `--experimental-codegen`, the
unique symbols check does not get run. this calculates the
`ir_node.unique_symbols` property, which implicitly runs the unique
symbols check.

vyper/venom/ir_node_to_venom.py

@@ -113,6 +113,8 @@
 
 # convert IRnode directly to venom
 def ir_node_to_venom(ir: IRnode) -> IRContext:
+    _ = ir.unique_symbols  # run unique symbols check
+
     global _global_symbols
     _global_symbols = {}