omp-low.c: Use more concrete types of gimple statement for various locals

omp-low.c: Use more concrete types of gimple statement for various locals

This corresponds to:
  [PATCH 47/89] omp-low.c: Use more concrete types of gimple statement for various locals
  https://gcc.gnu.org/ml/gcc-patches/2014-04/msg01203.html
from the original 89-patch kit

That earlier patch was approved by Jeff:
> OK once prerequisites have gone in.
in https://gcc.gnu.org/ml/gcc-patches/2014-05/msg00810.html

gcc/
	* omp-low.c (finalize_task_copyfn): Strengthen local "bind" from
	plain gimple to gimple_bind.
	(lower_rec_input_clauses): Strengthen local "g" from
	plain gimple to gimple_assign.
	(lower_lastprivate_clauses): Likewise for "stmt" to gimple_cond
	and "g" to gimple_call.
	(expand_omp_for_init_vars): Likewise, for two decls of "stmt" to
	gimple_assign.
	(expand_omp_atomic_pipeline): Likewise for one decl of "stmt".
	(expand_omp_atomic_mutex): Likewise.
	(lower_omp_master): Likewise for "x" to gimple_call.
	(lower_omp_ordered): Likewise.