nir: fix bug with moves in nir_opt_remove_phis()

nir: fix bug with moves in nir_opt_remove_phis()

In 144cbf8 ("nir: Make nir_opt_remove_phis see through moves."), Ken
made nir_opt_remove_phis able to coalesce phi nodes whose sources are
all moves with the same swizzle. However, he didn't add the logic
necessary for handling the fact that the phi may now have multiple
different sources, even though the sources point to the same thing. For
example, if we had something like:

if (...)
   a1 = b.yx;
else
   a2 = b.yx;
a = phi(a1, a2)
... = a

then we would rewrite it to

if (...)
   a1 = b.yx;
else
   a2 = b.yx;
... = a1

by picking a random phi source, which in this case is invalid because
the source doesn't dominate the phi. Instead, we need to change it to:

if (...)
   a1 = b.yx;
else
   a2 = b.yx;
a3 = b.yx;
... = a3;

Fixes 12 CTS tests:
ES31-CTS.functional.tessellation.invariance.outer_edge_symmetry.quads*

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>