[mlir][Interfaces] `Variable` abstraction for `ValueBoundsOpInterface` (#87980)

This commit generalizes and cleans up the `ValueBoundsConstraintSet`
API. The API used to provide function overloads for comparing/computing
bounds of:
- index-typed SSA value
- dimension of shaped value
- affine map + operands

This commit removes all overloads. There is now a single entry point for
each `compare` variant and each `computeBound` variant. These functions
now take a `Variable`, which is internally represented as an affine map
and map operands.

This commit also adds support for computing bounds for an affine map +
operands. There was previously no public API for that.