simplifier: use resultt<> instead of bool

[kroening]

The consistent use of resultt<> makes the code easier to read.

• 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.
• 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 #6885 (3e07b01) into develop (b1f440a) will increase coverage by 0.00%.
The diff coverage is 78.11%.

@@           Coverage Diff            @@
##           develop    #6885   +/-   ##
========================================
  Coverage    77.80%   77.80%           
========================================
  Files         1567     1567           
  Lines       179920   179928    +8     
========================================
+ Hits        139992   140000    +8     
  Misses       39928    39928           

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_internal_additions.cpp	90.12% <ø> (ø)
src/ansi-c/ansi_c_language.h	75.00% <ø> (ø)
src/ansi-c/expr2c.cpp	67.63% <ø> (ø)
.../goto-instrument/accelerate/acceleration_utils.cpp	2.23% <0.00%> (ø)
... and 223 more

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[tautschnig]

I had already implemented this in #6118.

[tautschnig]

Closing as this has been taken care of in #6118.