[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/D119153