radv/ac: gather4 cube workaround integer

radv/ac: gather4 cube workaround integer

This fix is extracted from amdgpu-pro shader traces.

It appears the gather4 workaround for integer types doesn't
work for cubes, so instead if forces a float scaled sample,
then converts to integer.

It modifies the descriptor before calling the gather.

This also produces some ugly asm code for reasons specified
in the patch, llvm could probably do better than dumping
sgprs to vgprs.

This fixes:
dEQP-VK.glsl.texture_gather.basic.cube.rgba8*

Acked-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Signed-off-by: Dave Airlie <airlied@redhat.com>