i965/vs: Split final assembly code generation out of vec4_visitor.

i965/vs: Split final assembly code generation out of vec4_visitor.

Compiling shaders requires several main steps:

   1. Generating VS IR from either GLSL IR or Mesa IR
   2. Optimizing the IR
   3. Register allocation
   4. Generating assembly code

This patch splits out step 4 into a separate class named "vec4_generator."

There are several reasons for doing so:

   1. Future hardware has a different instruction encoding.  Splitting
      this out will allow us to replace vec4_generator (which relies
      heavily on the brw_eu_emit.c code and struct brw_instruction) with
      a new code generator that writes the new format.

   2. It reduces the size of the vec4_visitor monolith.  (Arguably, a lot
      more should be split out, but that's left for "future work.")

   3. Separate namespaces allow us to make helper functions for
      generating instructions in both classes: ADD() can exist in
      vec4_visitor and create IR, while ADD() in vec4_generator() can
      create brw_instructions.  (Patches for this upcoming.)

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>