[CONTRACTS] Refactor quantified symbol detection for loop contracts

[qinheping]

Resolves: #8298

In loop contracts, we replace quantified variables with a tmp variable and add the tmp variable to the symbol map. However, the detection of the quantified variables is not complete now---it does not include equal expression.
This PR add equal expressions

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.32%. Comparing base (c320360) to head (a7383d9).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8299      +/-   ##
===========================================
- Coverage    78.35%   78.32%   -0.04%     
===========================================
  Files         1721     1721              
  Lines       188278   188338      +60     
  Branches     18460    18452       -8     
===========================================
- Hits        147523   147511      -12     
- Misses       40755    40827      +72     

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[remi-delmas-3000]

We don't seem to have tests for all the possible operators matched in add_quantified_variable (ternary op?)

[remi-delmas-3000]

Would it be possible to transform forall and exists nodes without having to explicitly match on other irep_ids ? Maybe using exprt::visit_post or exprt::visit_pre ?

[qinheping]

Rewrote the whole function with visit_pre.