hlsl: Return hlsl_ir_node pointers from instruction constructors, part 1.

The motivation for this is twofold.

Firstly, this makes code a little nicer to read, by virtue of removing a ubiquitous &(...)->node.

Secondly, and more importantly, this allows for the possibility of constructors returning a different instruction type than intended. The ultimate goal here is to return a preallocated "error" instruction when allocation fails, instead of returning NULL and propagating out-of-memory handling to the caller.