freedreno/ir3: add a pass to lower tg4 to txl, enable gather on a4xx

freedreno/ir3: add a pass to lower tg4 to txl, enable gather on a4xx

Unfortunately Adreno A4xx hardware returns incorrect results with the
GATHER4 opcodes. As a result, we have to lower to 4 individual texture
calls (txl since we have to force lod to 0). We achieve this using
offsets, including on cube maps which normally never have offsets.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Rob Clark <robdclark@gmail.com>