i965/fs: Add support for ir_tg4

i965/fs: Add support for ir_tg4

Lowers ir_tg4 (from textureGather and textureGatherOffset builtins) to
SHADER_OPCODE_TG4.

The usual post-sampling swizzle workaround can't work for ir_tg4,
so avoid doing that:

* For R/G/B/A swizzles use the hardware channel select (lives in the
   same dword in the header as the texel offset), and then don't do
   anything afterward in the shader.
* For 0/1 swizzles blast the appropriate constant over all the output
   channels instead of sampling.

V2: Avoid duplicating header enabling block
V3: Avoid sampling at all, for degenerate swizzles.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>