simplify_byte_extract: avoid unnecessary calls to has_subtype

[tautschnig]

This is a performance-improving refactoring: We only need the result of a has_subtype call under conditions that are infrequently met. On the benchmark of #7357, this avoids 443214 calls of has_subtype, which previously was the most costly part of simplify_byte_extract.

• 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).
• 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

Patch coverage: 100.00% and project coverage change: +0.02 🎉

Comparison is base (8c152bc) 78.53% compared to head (ff7af47) 78.55%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7712      +/-   ##
===========================================
+ Coverage    78.53%   78.55%   +0.02%     
===========================================
  Files         1686     1686              
  Lines       192930   192931       +1     
===========================================
+ Hits        151520   151562      +42     
+ Misses       41410    41369      -41     

Impacted Files	Coverage Δ
src/util/simplify_expr.cpp	85.50% <100.00%> (+<0.01%)	⬆️

... and 3 files with indirect coverage changes

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