aco: Mark VCC clobbered for iadd8 and iadd16 reductions on GFX6-7.

On GFX6-7, the 8 and 16-bit integer add reductions use the 32-bit v_add
instruction, which clobbers the VCC register.

Cc: mesa-stable
Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <!10346>