Make gimple_phi_arg_edge require a gimple_phi

Make gimple_phi_arg_edge require a gimple_phi

This corresponds to:
  [PATCH 52/89] Make gimple_phi_arg_edge require a gimple_phi
  https://gcc.gnu.org/ml/gcc-patches/2014-04/msg01228.html
from the original 89-patch kit

That earlier patch was approved by Jeff:
> Fine once prereqs go in.
in https://gcc.gnu.org/ml/gcc-patches/2014-05/msg00877.html

gcc/
	* gimple.h (gimple_phi_arg_edge): Require a gimple_phi rather
	than a plain gimple.

	* gimple-ssa-strength-reduction.c (ncd_with_phi): Strengthen
	param "phi" from gimple to gimple_phi.  Add a checked cast.
	(ncd_of_cand_and_phis): Add a checked cast.

	* graphite-sese-to-poly.c (rewrite_phi_out_of_ssa): Require a
	gimple_phi_iterator; strengthen local "phi" from gimple to a
	gimple_phi.
	(rewrite_cross_bb_scalar_deps): Strengthen local "psi" from
	a gimple_stmt_iterator to a gimple_phi_iterator.
	(edge_initial_value_for_loop_phi): Require a gimple phi.
	(initial_value_for_loop_phi): Likewise.

	* ipa-split.c (consider_split): Convert "bsi" to a
	gimple_phi_iterator and "stmt" to a gimple_phi.

	* predict.c (predict_extra_loop_exits): Convert "phi_stmt" to be
	a gimple_phi; introduce "lhs_def_stmt" as plain gimple.
	(apply_return_prediction): Convert "phi" to be a gimple_phi.

	* tree-cfg.c (replace_uses_by): Add checked cast to gimple_phi.
	(verify_gimple_in_cfg): Introduce gimple_phi_iterator "gpi" and use
	it to convert "phi" to a gimple_phi.

	* tree-eh.c (cleanup_empty_eh_merge_phis): Convert "ngsi", "ogsi"
	to be gimple_phi_iterators.  Convert "ophi", "nphi" to be
	gimple_phi.

	* tree-into-ssa.c (prepare_use_sites_for): Add checked cast to
	gimple_phi.

	* tree-ssa-coalesce.c (create_outofssa_var_map): Introduce
	gimple_phi_iterator "gpi" and use it to convert "phi" to a
	gimple_phi.

	* tree-ssa-dce.c (propagate_necessity): Introduce local "phi",
	from checked cast to gimple_phi.

	* tree-ssa-live.c (set_var_live_on_entry): Add checked cast to
	gimple_phi.

	* tree-ssa-propagate.c (replace_phi_args_in): Require a gimple_phi
	rather than a plain gimple.
	(substitute_and_fold_dom_walker::before_dom_children): Introduce
	gimple_phi_iterator "gpi".

	* tree-ssa-sink.c (find_bb_for_arg): Require a gimple_phi rather
	than a plain gimple.
	(nearest_common_dominator_of_uses): Replace check of code against
	GIMPLE_PHI with a dyn_cast<gimple_phi>, introducing a new local.
	(statement_sink_location): Add checked cast to gimple_phi.

	* tree-ssa-uninit.c (compute_uninit_opnds_pos): Require a
	gimple_phi rather than a plain gimple.
	(collect_phi_def_edges): Likewise.  Add a checked cast.
	(find_def_preds): Strengthen param "phi" from gimple to
	gimple_phi.
	(prune_uninit_phi_opnds_in_unrealizable_paths): Likewise for
	params "phi" and "flag_def".  Strenghen param "visited_phis" from
	hash_set<gimple> * to hash_set<gimple_phi> *.  Convert
	"flag_arg_def", "phi_arg_def" to gimple_phi using
	dyn_cast<gimple_phi>.  Similarly, introduce new local
	"opnd_def_phi".
	(use_pred_not_overlap_with_undef_path_pred): Strengthen param
	"phi" from gimple to gimple_phi, and param "visited_phis" from
	hash_set<gimple> * to hash_set<gimple_phi> *.  Add a checked cast.
	(is_use_properly_guarded): Likewise for params.
	(find_uninit_use): Replace check of code against GIMPLE_PHI with
	a dyn_cast<gimple_phi>, introducing a new local "use_phi".
	Strengthen local "visited_phis" from hash_set<gimple> to
	hash_set<gimple_phi>.