i965: Create a new vec4 backend for Broadwell.

i965: Create a new vec4 backend for Broadwell.

This replaces the old vec4_generator backend.

v2: Port to use the C-based instruction representation.  Also, remove
    Geometry Shader offset hacks - the visitor will handle those instead
    of this code.

v3: Texturing fixes (including adding textureGather support).

v4: Pass brw_context to gen8_instruction functions as required.

v5: Add SHADER_OPCODE_TXF_MCS support; port DUAL_INSTANCED gs fixes
    (caught by Eric).  Simplify ADDC/SUBB handling; add comments to
    gen8_set_dp_message calls (suggested by Matt).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Matt Turner <mattst88@gmail.com>