[mlir][Affine] Refactor affine fusion code in pass to utilities

Refactoring/clean-up step needed to add support for producer-consumer fusion
with multi-store producer loops and, in general, to implement more general
loop fusion strategies in Affine. It introduces the following changes:
  - AffineLoopFusion pass now uses loop fusion utilities more broadly to compute
    fusion legality (canFuseLoops utility) and perform the fusion transformation
    (fuseLoops utility).
  - Loop fusion utilities have been extended to deal with AffineLoopFusion
    requirements and assumptions while preserving both loop fusion utilities and
    AffineLoopFusion current functionality within a unified implementation.
    'FusionStrategy' has been introduced for this purpose and, in the future, it
    will allow us to have a single loop fusion core implementation that will produce
    different fusion outputs depending on the strategy used.
  - Improve separation of concerns for legality and profitability analysis:
    'isFusionProfitable' no longer filters out illegal scenarios that 'canFuse'
    didn't detect, or the other way around. 'canFuse' now takes loop dependences
    into account to determine the fusion loop depth (producer-consumer fusion only).
  - As a result, maximal fusion now doesn't require any profitability analysis.
  - Slices are now computed only once and reused across the legality, profitability
    and fusion transformation steps (producer-consumer).
  - Refactor some utilities and remove redundant copies of them.

This patch is NFCI and should preserve the existing functionality of both the
AffineLoopFusion pass and the affine fusion utilities.

Reviewed By: andydavis1, bondhugula

Differential Revision: https://reviews.llvm.org/D90798