i965/vec4: Fix broken saturate mask check in copy propagation.

i965/vec4: Fix broken saturate mask check in copy propagation.

try_copy_propagate() was checking the bit of the saturate mask for the
arg-th component of the source to decide whether the whole source
should be saturated (WTF?).  We need to swizzle the original saturate
mask and check that for all enabled channels the saturate flag is
either set or unset, as we cannot saturate a subset of destination
components only.

Reviewed-by: Abdiel Janulgue <abdiel.janulgue@linux.intel.com>