[mlir][SCF] Further simplify affine maps during `for-loop-canonicalization`

- Implement
`FlatAffineConstraints::getConstantBound(EQ)`. - Inject a simpler constraint for loops that have at most 1 iteration.
- Taking into account constant EQ bounds of FlatAffineConstraint dims/symbols during canonicalization of the resulting affine map in
`canonicalizeMinMaxOp`.

Differential Revision: https://reviews.llvm.org/D114138