nir: Add a phi node placement helper

nir: Add a phi node placement helper

Right now, we have phi placement code in two places and there are other
places where it would be nice to be able to do this analysis.  Instead of
repeating it all over the place, this commit adds a helper for placing all
of the needed phi nodes for a value.

v2: Add better documentation

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>