Assignment side effects may require additional temporaries

[tautschnig]

An assignment may invalidate pointers used in the left-hand side. In
case the result of the assignment is used, we therefore cannot re-use
the left-hand side to represent the result. Instead, introduce
temporaries as needed to store the right-hand side value, and use the
temporary to represent the result.

Fixes: #5857

• 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).
• n/a 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 #5858 (a59f36b) into develop (c4db021) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #5858   +/-   ##
========================================
  Coverage    72.89%   72.89%           
========================================
  Files         1423     1423           
  Lines       154229   154243   +14     
========================================
+ Hits        112426   112440   +14     
  Misses       41803    41803           

Impacted Files	Coverage Δ
src/goto-programs/goto_convert_class.h	87.30% <ø> (ø)
src/goto-programs/goto_clean_expr.cpp	99.48% <100.00%> (ø)
src/goto-programs/goto_convert_side_effect.cpp	93.10% <100.00%> (+0.31%)	⬆️

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