goto-symex: apply_condition should not change L2 index

[tautschnig]

Updating the L2 index would result in additional phi assignments, which
can result in more costly verification. One such example is that the use
of i != size instead of i < size in loop conditions resulted
increased verification time when we would expect not-equal to be cheaper
than less-than.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Attention: Patch coverage is 92.30769% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 78.35%. Comparing base (1ed7b2f) to head (3e18c42).

Files	Patch %	Lines
src/goto-symex/goto_symex.cpp	83.33%	3 Missing ⚠️
src/goto-symex/goto_state.cpp	66.66%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #6993      +/-   ##
===========================================
- Coverage    78.38%   78.35%   -0.04%     
===========================================
  Files         1720     1720              
  Lines       187982   188084     +102     
  Branches     18474    18478       +4     
===========================================
+ Hits        147346   147366      +20     
- Misses       40636    40718      +82     

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