Concretize three gimple_return_ accessors

Concretize three gimple_return_ accessors

This corresponds to:
  [PATCH 68/89] Concretize three gimple_return_ accessors
  https://gcc.gnu.org/ml/gcc-patches/2014-04/msg01233.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/msg00865.html

gcc/
	* gimple.h (gimple_return_retval_ptr): Require a const_gimple_return
	rather than a const_gimple.
	(gimple_return_retval): Likewise.
	(gimple_return_set_retval): Require a gimple_return.

	* cfgexpand.c (expand_gimple_stmt_1): Add a checked cast to
	gimple_return.
	(expand_gimple_basic_block): Likewise.
	* tree-complex.c (expand_complex_move): Likewise.
	(expand_complex_comparison): Likewise.
	* tree-inline.c (remap_gimple_stmt): Likewise.
	* tree-sra.c (scan_function): Likewise.
	(sra_modify_function_body): Likewise.
	(ipa_sra_modify_function_body): Likewise.
	* tree-ssa-structalias.c (find_func_aliases): Likewise.

	* gimple-ssa-isolate-paths.c (isolate_path): Strengthen local
	"ret" from gimple to gimple_return.
	(find_implicit_erroneous_behaviour): Replace a check for code
	GIMPLE_RETURN with a dyn_cast and a new local.
	(find_explicit_erroneous_behaviour): Likewise.
	* gimple-walk.c (walk_stmt_load_store_addr_ops): Likewise.
	* gimple.c (infer_nonnull_range): Likewise.
	* ipa-split.c (find_return_bb): Likewise.
	(find_retval): Likewise.
	(split_function): Likewise.
	* omp-low.c (ipa_simd_modify_function_body): Likewise.
	* tree-cfg.c (pass_warn_function_return::execute): Likewise.
	* tree-nrv.c (tree_nrv): Likewise.
	* tree-ssa-alias.c (ref_maybe_used_by_stmt_p): Likewise.
	* tree-ssa-dce.c (propagate_necessity): Likewise.
	* tree-ssa-structalias.c (find_func_clobbers): Likewise.
	* tree-tailcall.c (find_tail_calls): Likewise.

	* predict.c (apply_return_prediction): Rework the search for
	return_stmt so that the latter can have type gimple_return.