[mlir] Add a postprocessing parameter in Pattern

This adds a parameter SupplementalPatterns in tablegen class Pattern for
postprocessing code. For example, this can be used to ensure ops are
placed in the correct device by copying the atttributes that decide
devicement placement in Tensorflow dialect to prevent performance
regression.

Reviewed By: jpienaar

Differential Revision: https://reviews.llvm.org/D157032