[Polly][Isl] Use isl::val::sub instead of isl::val::sub_ui. NFC

This is part of an effort to reduce the differences between the custom C++ bindings used right now by polly in `lib/External/isl/include/isl/isl-noxceptions.h` and the official isl C++ interface.

Changes made:
 -  Use `isl::val::sub` instead of `isl::val::sub_ui`
 - `isl-noexceptions.h` has been generated by https://github.com/patacca/isl/commit/355e84163ae78ff637c71fb532f36d15277a2b1b

Depends on D107225

Reviewed By: Meinersbur

Differential Revision: https://reviews.llvm.org/D107293