[MLIR] FlatAffineConstraints: Fixed bug where some divisions were not being detected

This patch fixes a bug in the existing implementation of detectAsFloorDiv,
where floordivs with numerator with non-zero constant term and floordivs with
numerator only consisting of a constant term were not being detected.

Reviewed By: vinayaka-polymage

Differential Revision: https://reviews.llvm.org/D107214