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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>