Convert remaining gimple_omp_single_ accessors to be typesafe
gcc/ChangeLog.gimple-classes:
* gimple.h (gimple_omp_single_clauses): Strengthen param from
const_gimple to const gomp_single *.
(gimple_omp_single_clauses_ptr): Strengthen param from gimple to
gomp_single *.
* gimple-walk.c (walk_gimple_op): Add checked cast.
* tree-inline.c (remap_gimple_stmt): Within case
GIMPLE_OMP_SINGLE, introduce local "omp_single_stmt" via a
checked cast and use in place of "stmt".
* tree-nested.c (convert_nonlocal_reference_stmt): Likewise.
(convert_local_reference_stmt): Likewise.