[mlir][SCF] Fix off-by-one bug in affine analysis

This change is NFC. There were two issues when passing/reading upper bounds into/from FlatAffineConstraints that negate each other, so the bug was not apparent. However, it made debugging harder because some constraints in the FlatAffineConstraints were off by one when dumping all constraints.

Differential Revision: https://reviews.llvm.org/D114137