Make all gimple_omp_return_ accessors typesafe
gcc/ChangeLog.gimple-classes:
* gimple.h (struct gimple_statement_omp_return): Rename to...
(struct gomp_return): ...this.
(is_a_helper <gimple_statement_omp_return *>::test): Rename to...
(is_a_helper <gomp_return *>::test): ...this.
(is_a_helper <const gimple_statement_omp_return *>::test): Rename to...
(is_a_helper <const gomp_return *>::test): ...this.
(gimple_build_omp_return): Strengthen return type from gimple to
gomp_return *.
(gimple_omp_return_set_nowait): Likewise for param.
(gimple_omp_return_set_lhs): Likewise.
(gimple_omp_return_lhs_ptr): Likewise.
(gimple_omp_return_nowait_p): Strengthen param from const_gimple
to const gomp_return *.
(gimple_omp_return_lhs): Likewise.
* gimple-pretty-print.c (dump_gimple_omp_return): Strengthen param
from gimple to gomp_return *.
(pp_gimple_stmt_1): Add checked cast.
* gimple-walk.c (walk_gimple_op): Likewise.
* gimple.c (gimple_build_omp_return): Strengthen return type and
local "p" from gimple to gomp_return *.
* omp-low.c (remove_exit_barrier): Replace check against
GIMPLE_OMP_RETURN with a dyn_cast, introducing local
"omp_return_stmt" and using it in place of "stmt".
(expand_omp_for_generic): Introduce local "omp_return_stmt" via a
checked cast, using it in place of "gsi_stmt (gsi)".
(expand_omp_for_static_nochunk): Likewise.
(expand_omp_for_static_chunk): Likewise.
(expand_omp_sections): Introduce local "omp_return_stmt" via a
checked cast, using it in place of "gsi_stmt (si)".
(expand_omp_single): Likewise.
(maybe_add_implicit_barrier_cancel): Strengthen local "omp_return"
from gimple to gomp_return * via a checked cast.