[mlir][linalg] Add a new pattern to handle folding unit reduction dims.

The output operands will be added to input operands if the generic op (on tensors)
becomes an elementwise operation. The outputs of the generic op is still the same.
They will be cleaned up by ReplaceWithEmptyTensorIfUnused pattern.

Reviewed By: mravishankar

Differential Revision: https://reviews.llvm.org/D138251