Use subclasses of gimple in various places

Use subclasses of gimple in various places

This corresponds to:
  [PATCH 31/89] Use subclasses of gimple in various places
  https://gcc.gnu.org/ml/gcc-patches/2014-04/msg01171.html
from the original 89-patch kit

That earlier patch was approved by Jeff:
> OK once any prerequisites are in and any renaming done.
in https://gcc.gnu.org/ml/gcc-patches/2014-05/msg00634.html

gcc/
	* asan.c (insert_if_then_before_iter): Require a gimple cond
	rathern than a plain gimple.
	(asan_expand_check_ifn): Add a checked cast to gimple_cond.

	* cfgloopmanip.c (create_empty_if_region_on_edge): Likewise.

	* omp-low.c (simd_clone_adjust): Strengthen local from gimple
	to gimple_phi.

	* sese.c (set_ifsese_condition): Strengthen local from gimple to
	gimple_cond.

	* tree-call-cdce.c (gen_one_condition): Strengthen locals from
	gimple to gimple_assign and gimple_cond.

	* tree-ssa-phiopt.c (minmax_replacement): Likewise.
	(cond_store_replacement): Strengthen locals from gimple to
	gimple_phi and gimple_assign.
	(cond_if_else_store_replacement_1): Likewise.

	* tree-ssa-pre.c (do_regular_insertion): Strengthen local from
	gimple to gimple_assign.

	* tree-switch-conversion.c (hoist_edge_and_branch_if_true):
	Strengthen local from gimple to gimple_cond.
	(gen_def_assigns): Return a gimple_assign rather than a plain
	gimple.
	(gen_inbound_check): Strengthen locals from gimple to gimple_cond
	and gimple_assign.

	* tree-vect-loop-manip.c (slpeel_add_loop_guard): Strengthen local
	from gimple to gimple_cond.
	(set_prologue_iterations): Strengthen locals from gimple to
	gimple_phi and gimple_cond.

	* value-prof.c (gimple_ic): Strengthen local from gimple to
	gimple_phi.
	(gimple_stringop_fixed_value): Strengthen locals from gimple to
	gimple_assign, gimple_cond, gimple_call, and gimple_phi.