[mlir][Linalg] NFC: Refactor fusion on tensors to enable extending

it to fusing different kinds of linalg operations on tensors.

The implementation of fusion on tensor was initially planned for just
GenericOps (and maybe IndexedGenericOps). With addition of
linalg.tensor_reshape, and potentially other such non-structured ops,
refactor the existing implementation to allow easier specification of
fusion between different linalg operations on tensors.

Differential Revision: https://reviews.llvm.org/D78463