Some users of is_parallel_of_n_reg_sets disregard the clobbers in a
parallel after it has returned "yes, this is a parallel of N sets and
maybe some clobbers".  But combine uses a clobber of const0_rtx to
indicate substitution failure, so this leads to disaster.

Fix this by checking for such special clobbers in is_parallel_of_n_reg_sets.


	PR rtl-optimization/68381
	* combine.c (is_parallel_of_n_reg_sets): Return false if the pattern
	is poisoned.

From-SVN: r230786