Introduce gimple_omp_critical

Introduce gimple_omp_critical

This corresponds to:
  [PATCH 37/89] Introduce gimple_omp_critical
  https://gcc.gnu.org/ml/gcc-patches/2014-04/msg01158.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/msg00809.html

gcc/
	* coretypes.h (gimple_omp_critical): New typedef.
	(const_gimple_omp_critical): New typedef.

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

	* gimple-walk.c (walk_gimple_op): Likewise.

	* gimple.c (gimple_build_omp_critical): Return a gimple_omp_critical
	rather than a plain gimple.
	(gimple_copy): Add checked casts to gimple_omp_critical
	within GIMPLE_OMP_CRITICAL case of switch statement.

	* gimple.h (gimple_debug): Likewise.
	(gimple_build_omp_critical): Return a gimple_omp_critical rather
	than a plain gimple.
	(gimple_omp_critical_name): Require a const_gimple_omp_critical
	rather than a plain const_gimple.
	(gimple_omp_critical_name_ptr): Require a gimple_omp_critical
	rather than a plain gimple.
	(gimple_omp_critical_set_name): Likewise.

	* omp-low.c (check_omp_nesting_restrictions): Add a checked cast
	to gimple_omp_critical within GIMPLE_OMP_CRITICAL case of switch
	statement, introducing a new local "other_crit" for type-safety.
	(lower_omp_critical): Strengthen local "stmt" to
	gimple_omp_critical.

	* tree-inline.c (remap_gimple_stmt): Add a checked cast to
	gimple_omp_critical within GIMPLE_OMP_CRITICAL case of switch
	statement.