Fix simplification getting rid of variable type

[leventeBajczi]

Fixes #472

[github-actions]

Benchexec test report for a selection of SV-Benchmarks (correct / incorrect / all):

Rundefinition	COMPLEX	EMERGENT	HORN
SV-COMP25_no-data-race	✅ (59 / 0 / 89) HTML/CSV	✅ (35 / 0 / 73) HTML/CSV	✅ (20 / 0 / 82) HTML/CSV
SV-COMP25_no-overflow	✅ (25 / 0 / 61) HTML/CSV	❓ (0 / 0 / 37) HTML/CSV	✅ (1 / 0 / 90) HTML/CSV
SV-COMP25_termination	✅ (1 / 0 / 205) HTML/CSV	✅ (1 / 0 / 202) HTML/CSV	✅ (6 / 0 / 279) HTML/CSV
SV-COMP25_unreach-call	✅ (18 / 0 / 94) HTML/CSV	✅ (16 / 0 / 88) HTML/CSV	✅ (9 / 0 / 191) HTML/CSV
SV-COMP25_valid-memcleanup	❓ (0 / 0 / 66) HTML/CSV	❓ (0 / 0 / 66) HTML/CSV	❓ (0 / 0 / 66) HTML/CSV
SV-COMP25_valid-memsafety	✅ (34 / 0 / 313) HTML/CSV	✅ (3 / 0 / 278) HTML/CSV	❓ (0 / 0 / 421) HTML/CSV

[csanadtelbisz]

LGTM, but please fix the tests before merging.