instructiont::get_code -> code

[kroening]

This renames the field code to _code, introduces instructiont::code() as a
replacement for get_code(), changes the uses of get_code() to code(), and marks
get_code as deprecated.

The rationale is to match the pattern of the other accessor methods of the
class.

• 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.
• 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.
• White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Merging #6857 (66a5a42) into develop (0de72ad) will decrease coverage by 0.00%.
The diff coverage is 64.57%.

@@             Coverage Diff             @@
##           develop    #6857      +/-   ##
===========================================
- Coverage    77.79%   77.78%   -0.01%     
===========================================
  Files         1567     1567              
  Lines       179713   179701      -12     
===========================================
- Hits        139803   139788      -15     
- Misses       39910    39913       +3     

Impacted Files	Coverage Δ
src/analyses/cfg_dominators.h	100.00% <ø> (ø)
src/analyses/flow_insensitive_analysis.cpp	74.64% <0.00%> (ø)
src/analyses/interval_analysis.cpp	0.00% <0.00%> (ø)
src/analyses/invariant_propagation.cpp	0.00% <0.00%> (ø)
src/analyses/invariant_set_domain.cpp	0.00% <0.00%> (ø)
src/analyses/static_analysis.cpp	46.69% <0.00%> (ø)
src/ansi-c/ansi_c_language.h	75.00% <ø> (ø)
.../goto-instrument/accelerate/acceleration_utils.cpp	2.22% <0.00%> (ø)
...o-instrument/accelerate/polynomial_accelerator.cpp	0.00% <0.00%> (ø)
...goto-instrument/accelerate/sat_path_enumerator.cpp	57.89% <0.00%> (ø)
... and 72 more

---

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