c++: DECL_DECOMPOSITION_P cleanup

c++: DECL_DECOMPOSITION_P cleanup

DECL_DECOMPOSITION_P already checks VAR_P but we repeat the check
in a lot of places.

gcc/cp/ChangeLog:

	* decl.cc (duplicate_decls): Don't check VAR_P before
	DECL_DECOMPOSITION_P.
	* init.cc (build_aggr_init): Likewise.
	* parser.cc (cp_parser_range_for): Likewise.
	(do_range_for_auto_deduction): Likewise.
	(cp_convert_range_for): Likewise.
	(cp_convert_omp_range_for): Likewise.
	(cp_finish_omp_range_for): Likewise.
	* pt.cc (extract_locals_r): Likewise.
	(tsubst_omp_for_iterator): Likewise.
	(tsubst_decomp_names): Likewise.
	(tsubst_stmt): Likewise.
	* typeck.cc (maybe_warn_about_returning_address_of_local): Likewise.