tree-optimization/108970 - ICE with vectorizer peeling

tree-optimization/108970 - ICE with vectorizer peeling

The function slpeel_can_duplicate_loop_p fails to verify we can
copy blocks, instead slpeel_tree_duplicate_loop_to_edge_cfg does
but that's too late.  The following fixes this, also simplifying
error reporting which is somewhat pointless if we ICE immediately.

	PR tree-optimization/108970
	* tree-vect-loop-manip.cc (slpeel_can_duplicate_loop_p):
	Check we can copy the BBs.
	(slpeel_tree_duplicate_loop_to_edge_cfg): Avoid redundant
	check.
	(vect_do_peeling): Streamline error handling.

	* gcc.dg/pr108970.c: New testcase.