i965: Create a new fragment shader backend for Broadwell.

i965: Create a new fragment shader backend for Broadwell.

This replaces the old fs_generator backend.

v2: Port to the C-based representation of assembly instructions.
    Fix texturing after the texture-grf merge.

v3: Add high quality derivative support.  Fix SET_SIMD4X2_OFFSET.

v4: Pass brw_context to gen8_instruction functions as required.

v5: Fixes for MRT, as well as zero render targets (alpha test only).

v6: Replace n-wide with SIMDn in comments and messages; port over
    Topi's blorp-generator changes; add missing TXF_MCS opcode,
    fix missing high quality derivatives for DDX; fix typo (all caught
    by Eric).  Simplify ADDC/SUBB handling; drop "Used only on Gen6+"
    comment (caught by Matt).  Emit SIMD16 versions of three source
    instructions (caught by both Eric and Matt).

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