tree-optimization/98736 - use programing order preserved RPO in ldist

Tree loop distribution uses RPO to build reduced dependence graph,
it's important that RPO preserves the original programing order.
Though it usually does so, when distributing loop nest, exit BB can
be placed before some loop BBs while after loop header.  This patch
fixes the issue by calling rev_post_order_and_mark_dfs_back_seme.

gcc/ChangeLog:

	PR tree-optimization/98736
	* tree-loop-distribution.c
	* (loop_distribution::bb_top_order_init):
	Compute RPO with programing order preserved by calling function
	rev_post_order_and_mark_dfs_back_seme.

gcc/testsuite/ChangeLog:

	PR tree-optimization/98736
	* gcc.c-torture/execute/pr98736.c: New test.