Introduce gimple_omp_atomic_store

Introduce gimple_omp_atomic_store

This corresponds to:
  [PATCH 35/89] Introduce gimple_omp_atomic_store
  https://gcc.gnu.org/ml/gcc-patches/2014-04/msg01227.html
from the original 89-patch kit

That earlier patch was approved by Jeff:
> OK with expected changes due to renaming/updates to const handling.
> Please repost the final patch for archival purposes.
in https://gcc.gnu.org/ml/gcc-patches/2014-05/msg00804.html

gcc/
	* coretypes.h (gimple_omp_atomic_store): New typedef.
	(const_gimple_omp_atomic_store): New typedef.

	* gimple-pretty-print.c (dump_gimple_omp_atomic_store): Require
	a gimple_omp_atomic_store rather than a plain gimple.
	(pp_gimple_stmt_1): Add checked cast to gimple_omp_atomic_store
	within GIMPLE_OMP_ATOMIC_STORE case of switch statement.
	* gimple-walk.c (walk_gimple_op): Likewise.

	* gimple.c (gimple_build_omp_atomic_store): Return a
	gimple_omp_atomic_store rather than a plain gimple.

	* gimple.h (gimple_build_omp_atomic_store): Return a
	gimple_omp_atomic_store rather than a plain gimple.
	(gimple_omp_atomic_store_set_val): Require a gimple_omp_atomic_store
	rather than a plain gimple.
	(gimple_omp_atomic_store_val_ptr): Likewise.
	(gimple_omp_atomic_store_val): Require a
	const_gimple_omp_atomic_store rather than a plain const_gimple.

	* gimplify.c (gimplify_omp_atomic): Strengthen locals "loadstmt" and
	"storestmt" from gimple to gimple_omp_atomic_load loadstmt and
	gimple_omp_atomic_store storestmt respectively.

	* omp-low.c (expand_omp_atomic): Strengthen local "store" from
	gimple to gimple_omp_atomic_store.