[mlir][Linalg] Allow collapsing subset of the reassociations when fusing by collapsing.

This change generalizes the fusion of `tensor.expand_shape` ->
`linalg.generic` op by collapsing to handle cases where only a subset
of the reassociations specified in the `tensor.expand_shape` are valid
to be collapsed.
The method that does the collapsing is refactored to allow it to be a
generic utility when required.

Reviewed By: gysit

Differential Revision: https://reviews.llvm.org/D123153

Author: Mahesh Ravishankar