i965/Gen4-5: Set clip flags from clip distances

i965/Gen4-5: Set clip flags from clip distances

V2: - Use the new VS_OPCODE_UNPACK_FLAGS_SIMD4X2 to correctly split the
      flags for the two vertices being processed together.
    - Don't apply bogus masking of clip flags. The set of plane enables
      aren't included in the shader key, and we wouldn't want the
      recompiles anyway.

V3: - Tidy up spurious instructions, name temps properly.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
[V2] Reviewed-by: Paul Berry <stereotype441@gmail.com>