[MLIR][Presburger] fix vector update in coalesce

When `addCoalescedPolyhedron` was called with `j == n - 1`,
the `polyhedrons`-vector was not properly updated (the
`IntegerPolyhedron` at position `n - 2` was "lost"). This patch adds
special handling to that case and a regression testcase.

Reviewed By: Groverkss

Differential Revision: https://reviews.llvm.org/D121356