vect: Fix dominator update [PR123152]

vect: Fix dominator update [PR123152]

The `recompute_dominator' function used in the code fragment within
this patch assumes correctness in the rest of the CFG. Consequently,
it is wrong to rely upon it before the subsequent updates are made in
the "Update dominators for multiple exits" loop in the function.

Furthermore, if `loop_exit' == `scalar_exit', the "Update dominators for
multiple exits" logic will already take care of updating the
dominator for `scalar_exit->dest', such that the moved statement is
unnecessary.

gcc/ChangeLog:

	PR tree-optimization/123152
	* tree-vect-loop-manip.cc
	(slpeel_tree_duplicate_loop_to_edge_cfg): Correct order of
	dominator update.

gcc/testsuite/ChangeLog:

	* gcc.dg/vect/vect-uncounted-prolog-peel_2.c: New.
	* gcc.dg/vect/vect-uncounted-prolog-peel_3.c: Likewise.
	* g++.dg/vect/vect-uncounted-prolog-peel_1.cc: Likewise.