fix accesses to exprt::opX() in pointer-analysis/

[kroening]

This improves type safety.

• 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).
• n/a 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.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 0c84e3d).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/123391575

[smowton]

Did #5025; @owen-jones-diffblue would you mind looking at this one?

[kroening]

all done

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: fc59f8f).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/123586384

[owen-mc-diffblue]

A lot of the comments weren't addressed. Perhaps you didn't click "Load more", which expands all the comments that Github hides when there are too many comments?

[allredj]

⚠️
This PR failed Diffblue compatibility checks (cbmc commit: c69ec8d).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/123936380
Status will be re-evaluated on next push.
Common spurious failures include: the cbmc commit has disappeared in the mean time (e.g. in a force-push); the author is not in the list of contributors (e.g. first-time contributors); compatibility was already broken by an earlier merge.

[owen-mc-diffblue]

Two things still left

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: f578d54).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/124073765

[codecov-io]

Codecov Report

Merging #5027 into develop will increase coverage by 0.02%.
The diff coverage is 73.38%.

@@             Coverage Diff             @@
##           develop    #5027      +/-   ##
===========================================
+ Coverage    69.53%   69.55%   +0.02%     
===========================================
  Files         1314     1314              
  Lines       108988   108970      -18     
===========================================
+ Hits         75786    75798      +12     
+ Misses       33202    33172      -30

Impacted Files	Coverage Δ
src/pointer-analysis/goto_program_dereference.cpp	25.71% <7.69%> (+0.48%)	⬆️
src/pointer-analysis/value_set_fi.cpp	61.28% <76.08%> (+1.98%)	⬆️
src/pointer-analysis/value_set.cpp	76.8% <84.61%> (+2.1%)	⬆️

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