i965/fs: add generator support for pixel interpolator query

V5: - Split into separate opcodes
    - Pass message data in src1 immediate
    - Put noperspective bit in fs_inst rather than adding any junk to
      backend_instruction

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
i965/fs: Skip channel expressions splitting for interpolation

The backend will have to do a message send, so we want to keep these in
one piece, just like texture ops.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
i965/fs: add support for ir_*_interpolate_at_* expressions

SIMD8-only for now.

V5: - Fix style complaints
    - Move prototype to be with other oddball emit functions
    - Use unreachable() instead of assert() where possible

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>