[MLIR] NFC. Expose/move MDG out of Affine fusion into Affine Analysis

Move out MemRefDependenceGraph analysis structure out of LoopFusion into
the Affine Analysis library. This had been a long pending TODO. Moving
MDG out allows its use in other affine passes as well as allows building
custom affine fusion passes downstream while reusing upstream fusion
utilties. The file LoopFusion.cpp had also become lengthy and this
change makes things more modular. This change is a pure NFC and is a
code movement.

NFC.

Reviewed By: springerm

Differential Revision: https://reviews.llvm.org/D147105