tree-predcom.c: use gimple_phi in various places

tree-predcom.c: use gimple_phi in various places

This corresponds to:
  [PATCH 12/89] tree-predcom.c: use gimple_phi in various places
  https://gcc.gnu.org/ml/gcc-patches/2014-04/msg01229.html
from the original 89-patch kit

That earlier patch was approved by Jeff:
> OK when prerequisites have gone in.
in https://gcc.gnu.org/ml/gcc-patches/2014-05/msg00621.html

gcc/
	* tree-predcom.c (find_looparound_phi): Return a gimple_phi rather
	than just a gimple.
	(insert_looparound_copy): Require a gimple_phi rather than just a
	gimple.
	(add_looparound_copies): Convert local "phi" to be a gimple_phi.
	(initialize_root_vars): Likewise.
	(initialize_root_vars_lm): Likewise.