Convert various gimple to gimple_phi within ssa-iterators.h

Convert various gimple to gimple_phi within ssa-iterators.h

This corresponds to:
  [PATCH 89/89] Convert various gimple to gimple_phi within ssa-iterators.h
  https://gcc.gnu.org/ml/gcc-patches/2014-04/msg01177.html
from the original 89-patch kit

That earlier patch was approved by Jeff:
> OK once prereq go in.
in https://gcc.gnu.org/ml/gcc-patches/2014-05/msg00861.html

gcc/
	* ssa-iterators.h (FOR_EACH_PHI_OR_STMT_USE): Add checked cast to
	gimple_phi.
	(FOR_EACH_PHI_OR_STMT_DEF): Likewise.

	* ssa-iterators.h (single_phi_def): Require a gimple_phi.
	(op_iter_init_phiuse): Likewise.
	(op_iter_init_phidef): Likewise.
	* tree-ssa-loop-im.c (extract_true_false_args_from_phi): Likewise.

	* tree-ssa-loop-im.c (link_use_stmts_after): Replace check against
	GIMPLE_PHI with add a dyn_cast to gimple_phi, using result as needed.
	(determine_max_movement): Likewise.
	* tree-ssa-reassoc.c (is_phi_for_stmt): Likewise.
	* tree-ssa-sccvn.c (start_over:): Likewise.

	* tree-ssa-loop-im.c
	(move_computations_dom_walker::before_dom_children): Split iterator
	into gimple_stmt_iterator and a gimple_phi_iterator so that a local
	can become a gimple_phi.