[mlir][Linalg] Add callbacks to fusion of elementwise operations to control fusion.

Right now Elementwise operations fusion in Linalg fuses everything it
can. This can run up against resource limits of the target hardware
without some checks. This patch adds a callback function that clients
can use to implement a cost function. When two elementwise operations
are deemed structurally fusable, the callback can be used to control
if the fusion applies.

Differential Revision: https://reviews.llvm.org/D99820