[mlir][Linalg] Split `populateElementwiseOpsFusionPatterns`.

The method to add elementwise ops fusion patterns pulls in many other
patterns by default. The patterns to pull in along with the
elementwise op fusion should be upto the caller. Split the method to
pull in just the elementwise ops fusion pattern. Other cleanup changes
include
- Move the pattern for constant folding of generic ops (currently only
  constant folds transpose) into a separate file, cause it is not
  related to fusion
- Drop the uber LinalgElementwiseFusionOptions. With the
  populateElementwiseOpsFusionPatterns being split, this has no
  utility now.
- Drop defaults for the control function.
- Fusion of splat constants with generic ops doesnt need a control
  function. It is always good to do.

Differential Revision: https://reviews.llvm.org/D123236