HLSL: struct splitting: assignments of hierarchical split types

HLSL: struct splitting: assignments of hierarchical split types

This commit adds support for copying nested hierarchical types of split
types.  E.g, a struct of a struct containing both user and builtin interstage
IO variables.

When copying split types, if any subtree does NOT contain builtin interstage
IO, we can copy the whole subtree with one assignment, which saves a bunch
of AST verbosity for memberwise copies of that subtree.