remove uses of typet::subtype() const

[kroening]

• Each commit message has a non-empty body, explaining why the change was made.
• 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]

I certainly approve of this in spirit (this doesn't compile just yet and needs a proper commit message).

[codecov]

Codecov Report

Merging #6618 (9073968) into develop (f9456fe) will increase coverage by 0.00%.
The diff coverage is 79.16%.

@@           Coverage Diff            @@
##           develop    #6618   +/-   ##
========================================
  Coverage    77.79%   77.79%           
========================================
  Files         1567     1567           
  Lines       179818   179841   +23     
========================================
+ Hits        139895   139913   +18     
- Misses       39923    39928    +5     

Impacted Files	Coverage Δ
...goto-harness/memory_snapshot_harness_generator.cpp	76.17% <0.00%> (ø)
src/goto-programs/string_instrumentation.cpp	25.11% <0.00%> (-0.06%)	⬇️
src/cpp/expr2cpp.cpp	42.97% <33.33%> (+0.28%)	⬆️
src/cpp/cpp_typecheck_conversions.cpp	84.97% <52.94%> (-0.18%)	⬇️
src/pointer-analysis/value_set_fi.cpp	69.82% <71.42%> (-0.02%)	⬇️
jbmc/src/java_bytecode/ci_lazy_methods_needed.cpp	98.41% <100.00%> (ø)
jbmc/src/java_bytecode/convert_java_nondet.cpp	79.72% <100.00%> (+0.27%)	⬆️
jbmc/src/java_bytecode/expr2java.cpp	86.25% <100.00%> (ø)
...bmc/src/java_bytecode/java_bytecode_parse_tree.cpp	9.90% <100.00%> (ø)
jbmc/src/java_bytecode/java_bytecode_parser.cpp	90.32% <100.00%> (ø)
... and 6 more

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