simplify prop_conv_solvert::push

[kroening]

This change relieves the user of prop_conv_solvert::push from the restriction that the expressions pushed as assumptions must be literal_exprt and frozen.

• 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

All modified and coverable lines are covered by tests ✅

Comparison is base (3a1272a) 79.09% compared to head (00e5975) 79.08%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8122      +/-   ##
===========================================
- Coverage    79.09%   79.08%   -0.01%     
===========================================
  Files         1697     1697              
  Lines       196490   196493       +3     
===========================================
- Hits        155407   155395      -12     
- Misses       41083    41098      +15     

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