Variables that contain more than one object (arrays or structs) require
the allocation of contiguous registers in the respective object
register spaces.
This patch makes index expressions on resources hlsl_ir_index nodes
instead of hlsl_ir_resource_load nodes, because it is not known if they
will be used later as the lhs of an hlsl_ir_resource_store.
For now, the only benefit is consistency.
From this point on, it is no longer true that only hlsl_ir_loads can
return objects, because an object can also come from chain of
hlsl_ir_indexes that ends in an hlsl_ir_load.
The lower_index_loads pass takes care of lowering all hlsl_ir_indexes
into hlsl_ir_loads.
For this reason, hlsl_resource_load_params now expects both the resource
as the sampler to be just an hlsl_ir_node pointer instead of a pointer
to a more specific hlsl_ir_load.
This node type is intended for use during parse-time.
While we parse an indexing expression such as "a[3]", we don't know if
it will end up as part of an expression (in which case it must be folded
into a load) or it is for the lhs of a store (in which case it must be
folded into the store's deref).
