[MLIR][Presburger] subtract: fix bug when an input set has duplicate divisions

Previously, when an input set had a duplicate division, the duplicates might
be removed by a call to mergeLocalIds due to being detected as being duplicate
for the first time. The subtraction implementation cannot handle existing
locals being removed, so this would lead to unexpected behaviour. Resolve this
by removing all the duplicates up front.

Reviewed By: Groverkss

Differential Revision: https://reviews.llvm.org/D122826