nvc0/ir: handle a load's reg result not being used for locked variants

nvc0/ir: handle a load's reg result not being used for locked variants

For a load locked, we might not use the first result but the second
result is the predicate result of the locking. In that case the load
splitting logic doesn't apply (which is designed for splitting 128-bit
loads). Instead we take the predicate and move it into the first
position (as having a dead result in first def's position upsets all
sorts of things including RA). Update the emitters to deal with this as
well.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Tested-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>