nir/regs_to_ssa: Handle regs in phi sources properly

Sources of phi instructions act as if they occur at the very end of the
predecessor block not the block in which the phi lives.  In order to
handle them correctly, we have to skip phi sources on the normal
instruction walk and handle them as a separate walk over the successor
phis.  While registers in phi instructions is a bit of an oddity it can
happen when we temporarily go out-of-SSA for control-flow manipulations.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=111075


Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
(cherry picked from commit 6fb685fe)