convenience methods for division expressions

[kroening]

This adds the obvious convenience methods (dividend/divisor) for two
division expressions.

• Each commit message has a non-empty body, explaining why the change was made.
• 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

Merging #6806 (5e46184) into develop (aed93c8) will increase coverage by 0.00%.
The diff coverage is 97.59%.

@@           Coverage Diff            @@
##           develop    #6806   +/-   ##
========================================
  Coverage    76.99%   76.99%           
========================================
  Files         1594     1594           
  Lines       184263   184316   +53     
========================================
+ Hits        141872   141923   +51     
- Misses       42391    42393    +2     

Impacted Files	Coverage Δ
src/solvers/smt2/smt2_conv.h	100.00% <ø> (ø)
...c/solvers/smt2_incremental/convert_expr_to_smt.cpp	84.53% <0.00%> (-0.35%)	⬇️
src/util/simplify_expr_class.h	90.47% <ø> (ø)
src/util/std_expr.h	93.18% <ø> (ø)
src/ansi-c/c_typecheck_expr.cpp	77.05% <100.00%> (-0.02%)	⬇️
src/ansi-c/goto_check_c.cpp	91.47% <100.00%> (ø)
src/solvers/flattening/bv_pointers.cpp	83.09% <100.00%> (ø)
src/solvers/smt2/smt2_conv.cpp	67.47% <100.00%> (-0.02%)	⬇️
src/util/pointer_expr.h	84.64% <100.00%> (+0.77%)	⬆️
src/util/pointer_predicates.cpp	93.10% <100.00%> (ø)
... and 3 more

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