[mlir] Create a generic reduction detection utility

[mlir] Create a generic reduction detection utility

This patch introduces a generic reduction detection utility that works
across different dialecs. It is mostly a generalization of the reduction
detection algorithm in Affine. The reduction detection logic in Affine,
Linalg and SCFToOpenMP have been replaced with this new generic utility.

The utility takes some basic components of the potential reduction and
returns: 1) the reduced value, and 2) a list with the combiner operations.
The logic to match reductions involving multiple combiner operations disabled
until we can properly test it.

Reviewed By: ftynse, bondhugula, nicolasvasilache, pifon2a

Differential Revision: https://reviews.llvm.org/D110303