[mlir] Adds affine loop fusion transformation function to LoopFusionUtils.

[mlir] Adds affine loop fusion transformation function to LoopFusionUtils.

Summary:
Adds affine loop fusion transformation function to LoopFusionUtils.
Updates TestLoopFusion utility to run loop fusion transformation until a fixed point is reached.
Adds unit tests to test the transformation.

Reviewers: bondhugula, dcaballe, nicolasvasilache

Reviewed By: bondhugula, dcaballe

Subscribers: Joonsoo, merge_guards_bot, mehdi_amini, rriddle, jpienaar, burmako, shauheen, antiagainst, arpith-jacob, mgester, lucyrfox, aartbik, liufengdb, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73190