[mlir][Affine] Enable fusion of loops with vector loads/stores

This patch enables affine loop fusion for loops with affine vector loads
and stores. For that, we only had to use affine memory op interfaces in
LoopFusionUtils.cpp and Utils.cpp so that vector loads and stores are
also taken into account.

Reviewed By: andydavis1, ftynse

Differential Revision: https://reviews.llvm.org/D80971