SMT2 back-end and solver: it's fp.isInfinite, not fp.isInf

[tautschnig]

We already had gotten this right in one place, but got in wrong in 5
other places.

Fixes: #1782

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• 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).
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[codecov]

Codecov Report

Merging #6960 (27ac304) into develop (f239925) will increase coverage by 0.03%.
The diff coverage is 87.09%.

@@             Coverage Diff             @@
##           develop    #6960      +/-   ##
===========================================
+ Coverage    77.79%   77.83%   +0.03%     
===========================================
  Files         1568     1568              
  Lines       180347   180361      +14     
===========================================
+ Hits        140309   140378      +69     
+ Misses       40038    39983      -55     

Impacted Files	Coverage Δ
src/solvers/smt2/smt2_parser.cpp	78.36% <33.33%> (+0.43%)	⬆️
src/solvers/lowering/byte_operators.cpp	92.60% <90.00%> (-0.11%)	⬇️
src/goto-programs/remove_function_pointers.cpp	92.79% <100.00%> (+0.09%)	⬆️
src/solvers/smt2/smt2_conv.cpp	68.87% <100.00%> (+0.97%)	⬆️
src/goto-checker/solver_factory.cpp	77.24% <0.00%> (+0.68%)	⬆️
src/util/ieee_float.h	87.65% <0.00%> (+2.46%)	⬆️
src/solvers/smt2/smt2_format.cpp	86.51% <0.00%> (+4.49%)	⬆️
src/solvers/flattening/boolbv_shift.cpp	89.65% <0.00%> (+10.34%)	⬆️
... and 1 more

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