nv50/ir: fix phi/union sources when their def has been merged

nv50/ir: fix phi/union sources when their def has been merged

In a situation where double-register values are used, the phi nodes can
still end up being u32 values. They all get merged into one RA node
though. When fixing up the merge (which comes after the phi node), the
phi node's def would get fixed, but not its sources which would remain
at the low register value.

This maintains the invariant that a phi node's defs and sources are
allocated the same register.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>