i965/vec4: Fix copy propagation for non-register-aligned regions.

i965/vec4: Fix copy propagation for non-register-aligned regions.

This prevents it from trying to propagate a copy through a
register-misaligned region.  MOV instructions with a misaligned
destination shouldn't be treated as a direct GRF copy, because they
only define the destination GRFs partially.  Also fix the interference
check implemented with is_channel_updated() to consider overlapping
regions with different register offset to interfere, since the
writemask check implemented in the function is only valid under the
assumption that the source and destination regions are aligned
component by component.

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