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