[mlir][TilingInterface] Add a method to generate scalar implementation of the op.

While The tiling interface provides a mechanism for operations to be
tiled into tiled version of the op (or another op at the same level of
abstraction), the `generateScalarImplementation` method added here is
the "exit point" after all transformations have been done. Ops that
implement this method are expected to generate IR that are directly
lowerable to backend dialects like LLVM or SPIR-V dialects.

Differential Revision: https://reviews.llvm.org/D130612