tree-optimization/100053 - fix predication in VN

This avoids doing optimistic dominance queries involving
non-executable backedges when validating recorded predicated values
in VN because we have no way to force re-evaluating validity when
optimistically not executable edges become executable later.

2021-04-13  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/100053
	* tree-ssa-sccvn.c (vn_nary_op_get_predicated_value): Do
	not use optimistic dominance queries for backedges to validate
	predicated values.
	(dominated_by_p_w_unex): Add parameter to ignore executable
	state on backedges.
	(rpo_elim::eliminate_avail): Adjust.

	* gcc.dg/torture/pr100053.c: New testcase.
	* gcc.dg/tree-ssa/ssa-fre-93.c: Likewise.