WIP: extract virtual functions from ir_variable visitor

Includes making a new function to handle variables of type
ir_var_system_value, which differ in VS and GS.

The following functions have been renamed to reflect their generic
purpose, rather than their VS-specific behaviour:

- emit_attribute_fixups() -> emit_prolog()
- emit_vertex_program_code() -> emit_program_code()
- emit_urb_writes() -> emit_thread_end()
WIP: move VS-specific functions to vs_vec4_visitor
WIP: move VS-specific data members to vs_vec4_visitor.
WIP: Generalize data structures pointed to by vec4_generator.

vec4_generator never used the vertex-shader-specific parts of these
data structures anyway.
WIP: rename vec4_generator::generate_vs_instruction