goto-convert: introduce assignment_lhs_needs_temporary

[kroening]

This introduces goto_convertt::assignment_lhs_needs_temporary, to be used
instead of needs_cleaning when determining whether to introduce a temporary
for the value of an assignment expression, compound assignment expression,
or pre- or post-increment or decrement operator.

This allows weakening the condition computed by needs_cleaning.

• Each commit message has a non-empty body, explaining why the change was made.
• 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 #7047 (e7039b0) into develop (438aad3) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #7047      +/-   ##
===========================================
- Coverage    77.88%   77.88%   -0.01%     
===========================================
  Files         1569     1569              
  Lines       180989   181001      +12     
===========================================
+ Hits        140962   140971       +9     
- Misses       40027    40030       +3     

Impacted Files	Coverage Δ
src/goto-programs/goto_clean_expr.cpp	99.52% <100.00%> (ø)
src/goto-programs/goto_convert_class.h	88.15% <100.00%> (+0.32%)	⬆️
src/goto-programs/goto_convert_side_effect.cpp	94.47% <100.00%> (-0.67%)	⬇️

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