[MLIR] Fix areIdsUnique in AffineStructures

This patch fixes a bug in areIdsUnique where it ignores the [start, end] range.

No test case is added since there are no use cases through IR from where it
can be tested, and it is hard to create a unittest since we do not currently
have Values in unittests.

Reviewed By: arjunp

Differential Revision: https://reviews.llvm.org/D124735