CONTRACTS: apply: use previously computed mode

[tautschnig]

There is no need to do repeated lookups.

• 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.

[SaswatPadhi]

Thanks Michael, LGTM

[codecov]

Codecov Report

Merging #6798 (5a58dff) into develop (b7c6692) will increase coverage by 0.07%.
The diff coverage is 89.43%.

@@             Coverage Diff             @@
##           develop    #6798      +/-   ##
===========================================
+ Coverage    76.90%   76.97%   +0.07%     
===========================================
  Files         1590     1593       +3     
  Lines       183969   184107     +138     
===========================================
+ Hits        141486   141724     +238     
+ Misses       42483    42383     -100     

Impacted Files	Coverage Δ
..._incremental/smt2_incremental_decision_procedure.h	75.00% <ø> (ø)
...c/solvers/smt2_incremental/convert_expr_to_smt.cpp	84.87% <76.74%> (+12.78%)	⬆️
src/solvers/smt2_incremental/object_tracking.cpp	97.87% <97.87%> (ø)
src/goto-instrument/contracts/contracts.cpp	93.82% <100.00%> (-0.02%)	⬇️
src/goto-programs/goto_function.cpp	100.00% <100.00%> (ø)
src/solvers/smt2_incremental/object_tracking.h	100.00% <100.00%> (ø)
...ncremental/smt2_incremental_decision_procedure.cpp	94.15% <100.00%> (+0.14%)	⬆️
...t/solvers/smt2_incremental/convert_expr_to_smt.cpp	100.00% <100.00%> (ø)
unit/solvers/smt2_incremental/object_tracking.cpp	100.00% <100.00%> (ø)
src/util/std_code.h	94.08% <0.00%> (+0.01%)	⬆️
... and 5 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1a4c66e...5a58dff. Read the comment docs.