Introduce gimple_omp_continue

Introduce gimple_omp_continue

This corresponds to:
  [PATCH 36/89] Introduce gimple_omp_continue
  https://gcc.gnu.org/ml/gcc-patches/2014-04/msg01205.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/msg00826.html

gcc/
	* coretypes.h (gimple_omp_continue): New typedef.
	(const_gimple_omp_continue): New typedef.

	* gimple.h (gimple_build_omp_continue): Return a
	gimple_omp_continue rather than a plain gimple.
	(gimple_omp_continue_control_def): Require a
	const_gimple_omp_continue rather than a plain const_gimple.
	(gimple_omp_continue_control_use): Likewise.
	(gimple_omp_continue_control_def_ptr): Require a gimple_omp_continue
	rather than a plain gimple.
	(gimple_omp_continue_set_control_def): Likewise.
	(gimple_omp_continue_control_use_ptr): Likewise.
	(gimple_omp_continue_set_control_use): Likewise.

	* gimple-pretty-print.c (dump_gimple_omp_continue): Require a
	gimple_omp_continue rather than a plain gimple.
	(pp_gimple_stmt_1): Add a checked cast to gimple_omp_continue
	within GIMPLE_OMP_CONTINUE case of switch statement.

	* gimple-walk.c (walk_gimple_op): Likewise, adding a new local.

	* gimple.c (gimple_build_omp_continue): Return a
	gimple_omp_continue rather than a plain gimple.

	* omp-low.c (gimple_build_cond_empty): Return a gimple_cond
	rather than a plain gimple.
	(expand_omp_for_generic): Split local "stmt" into "assign_stmt",
	"cont_stmt", "cond_stmt", "call_stmt" of types gimple_assign,
	gimple_omp_continue, gimple_cond, gimple_call respectively.
	(expand_omp_for_static_nochunk): Likewise, splitting into two
	"cond_stmt" decls. "assign_stmt", "cont_stmt"
	(expand_omp_for_static_chunk): Likewise, splitting into
	"cond_stmt", "assign_stmt", "cont_stmt".
	(expand_omp_sections): Strengthen local "cont" from gimple to
	gimple_omp_continue.