[MLIR][Presburger][NFC] Use getters for IntegerPolyhedron members

This patch makes IntegerPolyhedron and derived classes use of getters to access
IntegerPolyhedron space information (`numIds, numDims, numSymbols`) instead of
directly accessing them.

This patch makes it easier to change the underlying implementation of the way
identifiers are stored, making it easier to extend/modify existing implementation.

Reviewed By: arjunp

Differential Revision: https://reviews.llvm.org/D118888