Move left-hand side type casts to right-hand side

[tautschnig]

Unlike goto_convertt::remove_assignment, the assignment handling code
in goto_convertt::clean_expr did not move type casts to the right-hand
side. This resulted in dump-c producing syntactically invalid code. The
modified code now uses the same approach as the ID_assign case of
goto_convertt::remove_assignment to avoid this problem.

This should address all failures of the CSmith CI job seen since March
2021.

• 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 #6888 (1faa1fe) into develop (9af369f) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #6888   +/-   ##
========================================
  Coverage    77.80%   77.80%           
========================================
  Files         1567     1567           
  Lines       179920   179921    +1     
========================================
+ Hits        139992   139993    +1     
  Misses       39928    39928           

Impacted Files	Coverage Δ
src/goto-programs/goto_clean_expr.cpp	99.52% <100.00%> (+<0.01%)	⬆️

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