[Polly][Isl] Use the function unsignedFromIslSize to manage a isl::size object. NFCI

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.
In the official interface the type `isl::size` cannot be casted to an unsigned without previously having checked if it contains a valid value with the function `isl::size::is_error()`.
For this reason two helping functions have been added:
 - `IslAssert`: assert that no errors are present in debug builds and just disables the mandatory error check in non-debug builds
 - `unisgnedFromIslSIze`: cast the `isl::size` object to `unsigned`

Changes made:
 - Add the functions `IslAssert` and `unsignedFromIslSize`
 - Add the utility function `rangeIslSize()`
 - Retype `MaxDisjunctsInDomain` from `int` to `unsigned`
 - Retype `RunTimeChecksMaxAccessDisjuncts` from `int` to `unsigned`
 - Retype `MaxDimensionsInAccessRange` from `int` to `uns...