Support shuffle_vector_exprt in goto programs

[tautschnig]

de4557f added support for _builtin_shuffle/__builtin_shufflevector via
a shuffle_vector_exprt. This expression, however, was lowered right away
in the C front end, making it unavailable to other language front-ends.
By moving the lowering to remove_vector there no longer is such a
limitation. Kani will be able to use it directly for Rust programs.

Regression test cbmc/gcc_vector3 continues to pass, demonstrating that
processing via the C front-end works as before.

Fixes: #6297

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• 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.
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[codecov]

Codecov Report

Merging #6645 (eec8e61) into develop (afc53d8) will increase coverage by 0.00%.
The diff coverage is 77.77%.

@@           Coverage Diff            @@
##           develop    #6645   +/-   ##
========================================
  Coverage    76.73%   76.73%           
========================================
  Files         1579     1579           
  Lines       181999   182012   +13     
========================================
+ Hits        139652   139665   +13     
  Misses       42347    42347           

Impacted Files	Coverage Δ
...rc/ansi-c/c_typecheck_gcc_polymorphic_builtins.cpp	76.98% <33.33%> (ø)
src/goto-programs/remove_vector.cpp	96.68% <100.00%> (+0.11%)	⬆️
src/ansi-c/c_expr.h	100.00% <0.00%> (ø)

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