i965/vec4: Check that the write offsets match when setting dependency controls.

i965/vec4: Check that the write offsets match when setting dependency controls.

For simplicity just assume that two writes to the same GRF with
different sub-GRF offsets will potentially interfere and break the
dependency control chain.  This is in preparation for adding sub-GRF
offset support to the VEC4 IR.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>