This is required in combination with the following commit, because
otherwise if a source region with an extended 8+ stride is present in
the instruction (which we're about to declare legal) we'll end up
emitting code that attempts to write to such a region, even though
strides greater than four are still illegal for the destination.
Tested-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
7f9f6263c1