Partly revert "Simplify initializer to remove byte_update"

[tautschnig]

This reverts commit ac31213, which can
break dump-c output: the simplified expression may have padding
initialised to non-zero values. This works fine for any analysis, but
breaks dump-c as padding is not included in the output.

Found via tests generated CSmith, a minimised version of which is
included as a regression test.

• 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.

[tautschnig]

Anyone running into CSmith failures on their PRs: this is the fix that needs to be merged to make them pass again.

[codecov]

Codecov Report

Merging #5780 (3a24545) into develop (5efb6ad) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #5780      +/-   ##
===========================================
+ Coverage    69.64%   69.68%   +0.03%     
===========================================
  Files         1248     1248              
  Lines       100920   100929       +9     
===========================================
+ Hits         70290    70331      +41     
+ Misses       30630    30598      -32     

Flag	Coverage Δ
cproversmt2	43.37% <100.00%> (+0.01%)	⬆️
regression	66.65% <100.00%> (+0.03%)	⬆️
unit	32.19% <0.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/ansi-c/c_typecheck_initializer.cpp	76.40% <100.00%> (ø)
src/goto-instrument/dump_c.cpp	79.65% <100.00%> (+2.62%)	⬆️
src/ansi-c/expr2c.cpp	70.53% <0.00%> (+1.32%)	⬆️

---

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 5efb6ad...3a24545. Read the comment docs.