Update various expressions within tree-scalar-evolution.c to be gimple_phi

Update various expressions within tree-scalar-evolution.c to be gimple_phi

This corresponds to:
  [PATCH 17/89] Update various expressions within tree-scalar-evolution.c to be gimple_phi
  https://gcc.gnu.org/ml/gcc-patches/2014-04/msg01150.html
from the original 89-patch kit

That earlier patch was approved by Jeff:
> OK when prerequisites have gone in.
> Actually that's true for #17 & #18 as well.
in https://gcc.gnu.org/ml/gcc-patches/2014-05/msg00626.html

gcc/
	* tree-scalar-evolution.c (follow_ssa_edge): Require a gimple_phi,
	rather than a gimple.
	(follow_ssa_edge_binary): Likewise.
	(follow_ssa_edge_expr): Likewise.
	(follow_ssa_edge_in_rhs): Likewise.
	(backedge_phi_arg_p): Likewise.
	(follow_ssa_edge_in_condition_phi_branch): Likewise.
	(follow_ssa_edge_in_condition_phi): Likewise.
	(follow_ssa_edge_inner_loop_phi): Likewise.
	(analyze_evolution_in_loop): Likewise.
	(analyze_initial_condition): Likewise.
	(interpret_loop_phi): Likewise.
	(interpret_condition_phi): Likewise.
	(follow_ssa_edge): Likewise; also, add checked casts to gimple_phi.

	(analyze_scalar_evolution_1): Add checked casts to gimple_phi
	within "case GIMPLE_PHI".