tree-optimization/114081 - dominator update for prologue peeling

The following implements manual update for multi-exit loop prologue
peeling during vectorization.

	PR tree-optimization/114081
	* tree-vect-loop-manip.cc (slpeel_tree_duplicate_loop_to_edge_cfg):
	Perform manual dominator update for prologue peeling.
	(vect_do_peeling): Properly update dominators after adding the
	prologue-around guard.

	* gcc.dg/vect/vect-early-break_121-pr114081.c: New testcase.