[mlir][tosa] Allow optional TOSA decompositions to be populated separately

Moved all TOSA decomposition patterns so that they can be optionally populated
and used by external rewrites. This avoids decomposing TOSa operations when
backends may benefit from the non-decomposed version.

Reviewed By: rsuderman, mehdi_amini

Differential Revision: https://reviews.llvm.org/D116526