use simplify_not instead of boolean_negate

[kroening]

This is follow-up on a discussion in #4982. boolean_negate performs some,
but not all, of the simplifications done by simplify_not. This PR avoids
this redundancy by calling simplify_not(not_exprt(x)) instead of
simplify_node(boolean_negate(x)).

• 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-io]

Codecov Report

Merging #4990 into develop will decrease coverage by <.01%.
The diff coverage is 85.71%.

@@             Coverage Diff             @@
##           develop    #4990      +/-   ##
===========================================
- Coverage    69.26%   69.26%   -0.01%     
===========================================
  Files         1309     1309              
  Lines       108424   108415       -9     
===========================================
- Hits         75104    75095       -9     
  Misses       33320    33320

Impacted Files	Coverage Δ
src/util/simplify_expr_if.cpp	34.23% <100%> (-1.17%)	⬇️
src/util/simplify_expr_boolean.cpp	88.73% <66.66%> (ø)	⬆️
src/util/simplify_expr_int.cpp	80.3% <87.5%> (-0.2%)	⬇️

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[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: c9b7fd2).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/122170679

[allredj]

Adding simplification steps in sensitive areas always makes me anxious. I wonder if we should be running a benchmark on this one, to check we're not taking a performance hit.