tree-optimization/95717 - fix SSA update for vectorizer epilogue

tree-optimization/95717 - fix SSA update for vectorizer epilogue

This fixes yet another issue with the custom SSA updating in the
vectorizer when we copy from the non-if-converted loop.  We must
not mess with current defs before we updated the BB copies.

2020-06-17  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/95717
	* tree-vect-loop-manip.c (slpeel_tree_duplicate_loop_to_edge_cfg):
	Move BB SSA updating before exit/latch PHI current def copying.

	* g++.dg/torture/pr95717.C: New testcase.

(cherry picked from commit d0909f5858ad81e6d8b73fa6193be19cb5e6ed7b)