[mlir][spirv] De-template serialization

Previously for each op we generate a separate serialization
method for it. Those serialization methods duplicate the logic
of parsing operands/results/attributes and such.

This commit creates a generic method and let suitable op-specific
serialization method to call into it.

wc -l SPIRVSerialization.inc: before 8304; after: 5597 (So -2707)

Reviewed By: hanchung, ThomasRaoux

Differential Revision: https://reviews.llvm.org/D93535