symex: use lhs/rhs instead of code_assignt

[kroening]

This replaces the use of code_assignt by an explicit lhs/rhs pair in goto
symex. This avoids constructing the code_assignt in a number of places.

• 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

Merging #5868 (e7429be) into develop (e0700db) will decrease coverage by 0.00%.
The diff coverage is 93.58%.

@@             Coverage Diff             @@
##           develop    #5868      +/-   ##
===========================================
- Coverage    72.90%   72.90%   -0.01%     
===========================================
  Files         1423     1423              
  Lines       154230   154206      -24     
===========================================
- Hits        112440   112419      -21     
+ Misses       41790    41787       -3     

Impacted Files	Coverage Δ
src/analyses/global_may_alias.cpp	0.00% <0.00%> (ø)
src/goto-diff/change_impact.cpp	0.00% <ø> (ø)
src/goto-programs/goto_convert_class.h	87.30% <ø> (ø)
src/goto-symex/auto_objects.cpp	21.95% <0.00%> (+0.52%)	⬆️
src/goto-symex/goto_symex.h	92.30% <ø> (ø)
src/goto-symex/symex_function_call.cpp	92.89% <50.00%> (+0.50%)	⬆️
src/goto-programs/goto_program.h	90.03% <66.66%> (ø)
src/analyses/constant_propagator.cpp	93.92% <100.00%> (-0.02%)	⬇️
src/analyses/custom_bitvector_analysis.cpp	54.97% <100.00%> (-0.12%)	⬇️
src/analyses/escape_analysis.cpp	64.31% <100.00%> (-0.47%)	⬇️
... and 22 more

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