Simplification towards singleton intervals requires single variable

[tautschnig]

We must not blindly assume that that and in >= C1 && !( >= C2) are the same.

Fixes: #7953

• 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).
• 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.

[NlightNFotis]

Oh shoot, did I cause this with #7761 ?

EDIT: Yeah, looks like I did - thanks for the quick fix.