use zero_extend_exprt in SMT2 front-end

[kroening]

This replaces the use of a typecast expression by zero_extend_exprt when parsing an SMT-LIB2 zero_extend expression.

• 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

All modified and coverable lines are covered by tests ✅

Project coverage is 78.88%. Comparing base (8fcd9b1) to head (6d8eb16).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8493   +/-   ##
========================================
  Coverage    78.88%   78.88%           
========================================
  Files         1727     1727           
  Lines       198398   198410   +12     
  Branches     18509    18528   +19     
========================================
+ Hits        156504   156518   +14     
+ Misses       41894    41892    -2     

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[tautschnig]

While at it: please change to id == ID_zero_extend