fix[venom]: add `unique_symbols` check to venom pipeline

[charles-cooper]

What I did

fix https://github.com/vyperlang/vyper/issues/4072

How I did it

How to verify it

Commit message

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.

also, change an assertion to a proper panic exception

Description for the changelog

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 90.35%. Comparing base (c79c0b6) to head (bbb89df).

:exclamation: Current head bbb89df differs from pull request most recent head dfe0040

Please upload reports for the commit dfe0040 to get more accurate results.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #4149 +/- ## ========================================== - Coverage 91.32% 90.35% -0.97% ========================================== Files 109 109 Lines 15569 15555 -14 Branches 3418 3415 -3 ========================================== - Hits 14218 14055 -163 - Misses 923 1040 +117 - Partials 428 460 +32 ```

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