R600/SI: Allow SI_KILL for geometry shaders

Reviewed-by: Tom Stellard <thomas.stellard@amd.com>
R600/SI: Optimize SI_KILL for constant operands

If the SI_KILL operand is constant, we can either clear the exec mask if
the operand is negative, or do nothing otherwise.

Reviewed-by: Tom Stellard <thomas.stellard@amd.com>