Fix trace invariant violation on smt2 incremental backend

[esteffin]

Fix invariant violation in incremental SMT solver trace generator.

An invariant was generated when the trace node was not a symbol_exprt, while now the expression is simplified by replacing its operands with the value got from the solver (as in the other backends).

This PR also fixes an inconsistency with the bit-vector shift operator when the left-hand-side operands has more bits than the left-hand.

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• 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).
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[codecov]

Codecov Report

Base: 78.38% // Head: 78.36% // Decreases project coverage by -0.02% ⚠️

Coverage data is based on head (50a8b01) compared to base (0841040).
Patch coverage: 97.50% of modified lines in pull request are covered.

❗ Current head 50a8b01 differs from pull request most recent head 10806ee. Consider uploading reports for the commit 10806ee to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7404      +/-   ##
===========================================
- Coverage    78.38%   78.36%   -0.03%     
===========================================
  Files         1647     1647              
  Lines       190362   189969     -393     
===========================================
- Hits        149213   148865     -348     
+ Misses       41149    41104      -45     

Impacted Files	Coverage Δ
...ncremental/smt2_incremental_decision_procedure.cpp	96.67% <94.11%> (-0.20%)	⬇️
src/solvers/smt2/smt2_conv.cpp	68.69% <100.00%> (+0.02%)	⬆️
...c/solvers/smt2_incremental/convert_expr_to_smt.cpp	83.48% <100.00%> (+0.10%)	⬆️
...t/solvers/smt2_incremental/convert_expr_to_smt.cpp	99.60% <100.00%> (+<0.01%)	⬆️
...ncremental/smt2_incremental_decision_procedure.cpp	98.59% <100.00%> (-0.17%)	⬇️
src/cpp/cpp_item.h	64.15% <0.00%> (-7.55%)	⬇️
src/cpp/cpp_typecheck_namespace.cpp	53.48% <0.00%> (-3.96%)	⬇️
src/goto-programs/goto_convert_exceptions.cpp	51.78% <0.00%> (-1.67%)	⬇️
src/cpp/cpp_typecheck_enum_type.cpp	77.19% <0.00%> (-1.32%)	⬇️
src/statement-list/statement_list_entry_point.cpp	83.87% <0.00%> (-1.28%)	⬇️
... and 146 more

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[thomasspriggs]

It would be good to have some regexes in these tests, to confirm that the generated traces are as we expect them to be.