Do not assign a non-ssa expression to object accessed via an ssa_exprt reference

[danpoe]

Previously a variable expr of type exprt & was cast to ssa_exprt & and assigned to variable ssa. Later the object was modified via assigning a non-ssa expression to expr, violating the invariants of ssa_exprt.

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[tautschnig]

@danpoe Please rebase now that #4729 is in to make CI happy.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (develop@e366841). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff             @@
##             develop    #4725   +/-   ##
==========================================
  Coverage           ?   68.45%           
==========================================
  Files              ?     1270           
  Lines              ?   104748           
  Branches           ?        0           
==========================================
  Hits               ?    71705           
  Misses             ?    33043           
  Partials           ?        0

Impacted Files	Coverage Δ
src/goto-symex/goto_symex_state.cpp	93.81% <100%> (ø)

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[allredj]

⚠️
This PR failed Diffblue compatibility checks (cbmc commit: 862aa15).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/113955224
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.