[PR105665] ivopts: check defs of names in base for undefs

[PR105665] ivopts: check defs of names in base for undefs

The patch for PR 100810 tested for undefined SSA_NAMEs appearing
directly in the base expression of the potential IV candidate, but
that's not enough.  The testcase for PR105665 shows an undefined
SSA_NAME has the same ill effect if it's referenced as an PHI_NODE arg
in the referenced SSA_NAME.  The variant of that test shows it can be
further removed from the referenced SSA_NAME.

To avoid deep recursion, precompute maybe-undefined SSA_NAMEs: start
from known-undefined nonvirtual default defs, and propagate them to
any PHI nodes reached by a maybe-undefined arg, as long as there
aren't intervening non-PHI uses, that would imply the maybe-undefined
name must be defined at that point, otherwise it would invoke
undefined behavior.  Also test for intervening non-PHI uses of DEFs in
the base expr.

The test for intervening uses implemented herein relies on dominance;
this could be further extended, regarding conditional uses in every
path leading to a point as an unconditional use dominating that point,
but I haven't implemented that.

for  gcc/ChangeLog

	PR tree-optimization/105665
	PR tree-optimization/100810
	* tree-ssa-loop-ivopts.cc
	(ssa_name_maybe_undef_p, ssa_name_set_maybe_undef): New.
	(ssa_name_any_use_dominates_bb_p, mark_ssa_maybe_undefs): New.
	(find_ssa_undef): Check precomputed flag and intervening uses.
	(tree_ssa_iv_optimize): Call mark_ssa_maybe_undefs.

for  gcc/testsuite/ChangeLog

	PR tree-optimization/105665
	PR tree-optimization/100810
	* gcc.dg/torture/pr105665.c: New.