[MLIR] The return type in the `computeSingleVarRepr` function is modified to...

[MLIR] The return type in the `computeSingleVarRepr` function is modified to include equality expressions.

Earlier `computeSingleVarRepr` was returning a pair of upper bound and
lower bound indices of the inequality contraints that can be expressed
as a floordiv of an affine function. The equality expression can also be
expressed as a floordiv but contains only one index and hence the `LocalRepr`
class is introduced to facilitate this.

Reviewed By: Groverkss

Differential Revision: https://reviews.llvm.org/D117430