Optimise propositional encoding of object_size

[tautschnig]

Avoid creating equalities over the postponed bitvector when the object
literals trivially aren't equal, and directly encode bitwise equality
when the object literals are trivially equal (and stop searching for a
matching object). In all other cases, avoid unnecessary Tseitin
variables to encode the postponed bitvector equality.

When running on various proofs done for AWS open-source projects, this
changes the performance as follows (when comparing to #7021): with
CaDiCaL as back-end, the total solver time for the hardest 46 proofs
changes from 26779.7 to 22409.9 seconds (4369.8 seconds speed-up); with
Minisat, however, the hardest 49 proofs take 28616.7 instead of 28420.4
seconds (196.3 seconds slow-down). Across these benchmarks, 11.7% of
variables and 12.8% of clauses are removed.

• 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

Merging #7029 (900e793) into develop (a326820) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #7029   +/-   ##
========================================
  Coverage    77.86%   77.86%           
========================================
  Files         1569     1569           
  Lines       180995   181002    +7     
========================================
+ Hits        140929   140936    +7     
  Misses       40066    40066           

Impacted Files	Coverage Δ
src/solvers/flattening/bv_utils.cpp	79.61% <ø> (ø)
src/solvers/flattening/bv_pointers.cpp	85.80% <100.00%> (+0.20%)	⬆️

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