[NFC][MLIR] Split large fusion test file into 4 test files

mlir/test/transforms/loop-fusion.mlir is too big and is split into mlir/test/transforms/loop-fusion.mlir,  mlir/test/transforms/loop-fusion-2.mlir, mlir/test/transforms/loop-fusion-3.mlir
and mlir/test/transforms/loop-fusion-4.mlir. Further tests can be added in mlir/test/transforms/loop-fusion-4.mlir

Reviewed By: bondhugula

Differential Revision: https://reviews.llvm.org/D106473