SMT2 back-end: flatten with_exprt operands

[tautschnig]

Solvers without datatypes support require flattening towards bitvectors. This is also true for array-typed expressions when the array is not at the top level (despite possible array-theory support). This changes makes sure we will flatten such array-typed expressions.

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[codecov]

Codecov Report

Attention: Patch coverage is 53.57143% with 13 lines in your changes missing coverage. Please review.

Project coverage is 80.39%. Comparing base (48490fb) to head (f1c3e8d).

Files with missing lines	Patch %	Lines
src/solvers/smt2/smt2_conv.cpp	53.57%	13 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8670      +/-   ##
===========================================
- Coverage    80.39%   80.39%   -0.01%     
===========================================
  Files         1688     1688              
  Lines       207387   207394       +7     
  Branches        73       73              
===========================================
+ Hits        166731   166733       +2     
- Misses       40656    40661       +5     

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