aco: only create parallelcopy to restore exec at loop exit if needed

aco: only create parallelcopy to restore exec at loop exit if needed

The operand isn't fixed to exec, which can mess up the spiller. This also
adds a new situation where a phi is needed.

Fixes dEQP-VK.ssbo.layout.random.descriptor_indexing.2 and an assertion
when compiling a Detroit: Become Human shader.

Fixes: 93c8ebfa ('aco: Initial commit of independent AMD compiler')
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/3257>