tree-optimization/100414 - compute dominance info in phiopt

phiopt now has dominator queries but fails to compute dominance
info.

2021-05-04  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/100414
	* tree-ssa-phiopt.c (get_non_trapping): Do not compute dominance
	info here.
	(tree_ssa_phiopt_worker): But unconditionally here.

	* gcc.dg/pr100414.c: New testcase.