SMT back-end: fix type when RHS will be flattened

[tautschnig]

When the right-hand side is a member expression the type has to be a bit vector (unless data types are used).

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• 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).
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[codecov]

Codecov Report

Base: 78.00% // Head: 78.00% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (09cd9ac) compared to base (062bc0c).
Patch coverage: 66.66% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #7252   +/-   ##
========================================
  Coverage    78.00%   78.00%           
========================================
  Files         1621     1621           
  Lines       187279   187278    -1     
========================================
+ Hits        146088   146089    +1     
+ Misses       41191    41189    -2     

Impacted Files	Coverage Δ
src/solvers/smt2/smt2_conv.cpp	65.39% <66.66%> (-0.01%)	⬇️
src/cprover/solver_progress.cpp	60.00% <0.00%> (-20.00%)	⬇️
src/cprover/solver.cpp	81.53% <0.00%> (-1.62%)	⬇️
src/cprover/propagate.cpp	73.52% <0.00%> (-1.48%)	⬇️
src/cprover/solver_types.h	88.88% <0.00%> (-0.59%)	⬇️
src/ansi-c/expr2c.cpp	67.27% <0.00%> (-0.05%)	⬇️
src/cprover/state_encoding.cpp	80.57% <0.00%> (+0.22%)	⬆️
src/cprover/axioms.cpp	92.11% <0.00%> (+0.54%)	⬆️
src/cprover/counterexample_found.cpp	93.33% <0.00%> (+11.19%)	⬆️

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