nir: Fix register handling in phi lowering
In some non-trivial cases, phi instruction has registers as the source, so we need to see to it.
Closes: #6370 (closed)
Fixes: !7587 (79a987ad) ("nir: CF optimizations involving break statements")
Signed-off-by: Mykhailo Skorokhodov mykhailo.skorokhodov@globallogic.com