Swapping goto_programt::instructiont must preserve labels

[tautschnig]

The swap method should maintain all information that cannot be computed.
(As those are computed, instruction numbers, incoming edges, target
numbers, loop numbers need not be preserved.)

• 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

Merging #6514 (6f7c25f) into develop (23d90c8) will increase coverage by 0.00%.
The diff coverage is 53.84%.

@@           Coverage Diff            @@
##           develop    #6514   +/-   ##
========================================
  Coverage    76.10%   76.10%           
========================================
  Files         1548     1548           
  Lines       166325   166326    +1     
========================================
+ Hits        126576   126587   +11     
+ Misses       39749    39739   -10     

Impacted Files	Coverage Δ
jbmc/src/jdiff/jdiff_parse_options.h	100.00% <ø> (ø)
src/analyses/goto_check.cpp	62.50% <ø> (ø)
src/ansi-c/ansi_c_internal_additions.cpp	90.36% <ø> (ø)
src/crangler/scanner.l	63.63% <ø> (ø)
src/goto-instrument/function_assigns.cpp	0.00% <0.00%> (ø)
src/goto-instrument/function_assigns.h	0.00% <0.00%> (ø)
src/goto-instrument/havoc_loops.cpp	0.00% <0.00%> (ø)
jbmc/src/java_bytecode/goto_check_java.cpp	31.47% <50.66%> (ø)
src/goto-instrument/loop_utils.cpp	82.85% <66.66%> (ø)
src/ansi-c/builtin_factory.cpp	66.66% <100.00%> (ø)
... and 16 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 718b547...6f7c25f. Read the comment docs.