[PR tree-optimization/78024] Clear basic block flags before using BB_VISITED for OpenACC loops processing

	gcc/
	* omp-low.c (oacc_loop_discovery): Call clear_bb_flags before, and
	don't clear BB_VISITED after processing.

	gcc/testsuite/
	* gcc.dg/goacc/loop-processing-1.c: New file.

From-SVN: r241334