Concretize gimple_assign_nontemporal_move_p

Concretize gimple_assign_nontemporal_move_p

This corresponds to:
  [PATCH 85/89] Concretize gimple_assign_nontemporal_move_p
  https://gcc.gnu.org/ml/gcc-patches/2014-04/msg01202.html
from the original 89-patch kit

That earlier patch was approved by Jeff:
> OK when prereqs have gone in.
in https://gcc.gnu.org/ml/gcc-patches/2014-05/msg00853.html

gcc/
	* gimple.h (gimple_assign_nontemporal_move_p): Require a
	const_gimple_assign rather than a const_gimple.

	* cfgexpand.c (expand_gimple_stmt_1): Add local assign_stmt and
	checked cast within "case GIMPLE_ASSIGN".

	* gimple-streamer-out.c (output_gimple_stmt): Add checked cast to
	gimple_assign.