aco: split self-intersecting copies instead of swapping

aco: split self-intersecting copies instead of swapping

Example situation:
v1 = {v0.hi, v1.lo}
v0.hi = v1.hi

The 4-byte copy's definition is completely used, but swapping it makes no
sense. We have to split it to generate correct code:
swap(v0.hi, v1.lo)
swap(v0.hi, v1.hi)

Found in dEQP-VK.spirv_assembly.type.vec3.i16.constant_composite_vert

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4772>