[DirectX] Represent instruction classes to encapsulate DXIL Op function signature

[bharadwajy]

A DXIL class represents a set of DXIL operations with the same function signature. Implement a TableGen representation of DXIL class in DXIL.td to encapsulate valid parameter types of DXIL ops.

Implement the functionality in TableGen backend, DXILEmitter, to generate necessary C++ data structures and/or tables for lowering passes to consume the attribute information.

[bharadwajy]

Using separate fields to represent DXIL Op arguments and result types and not using DXIL Op classes to encapsulate them as function signatures - see #95807.