LoopFusion: Creates private MemRefs which are used only by operations in the fused loop.

*) Enables reduction of private memref size based on MemRef region accessed by fused slice.
*) Enables maximal fusion by creating a private memref to break a fusion-preventing dependence.
*) Adds maximal fusion flag to enable fusing as much as possible (though it still fuses the minimum cost computation slice).

PiperOrigin-RevId: 229936698