fix[ir]: unique symbol name

[trocher]

What I did

STORE() unique symbol name is computed as _freshname(_freshname(f"{op}_")) instead of _freshname(f"{op}_") after #3835. This PR fixes this.

How I did it

Straightforward

How to verify it

Compiling a contract with a single SSTORE will lead to an IR containing [unique_symbol, sstore_1] where it would previously contain [unique_symbol, sstore_12].

Commit message

fix: unique symbol name

Description for the changelog

Fix unique symbol name

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.93%. Comparing base (787c562) to head (31d5465).

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3848      +/-   ##
==========================================
- Coverage   86.15%   85.93%   -0.22%     
==========================================
  Files          92       92              
  Lines       13921    13921              
  Branches     3067     3067              
==========================================
- Hits        11993    11963      -30     
- Misses       1491     1520      +29     
- Partials      437      438       +1     

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[charles-cooper]

thanks!