Reset PHI base0 flag if it's clear in any argument [PR101977, ...]

Reset PHI base0 flag if it's clear in any argument [PR101977, ...]

Resolves:
PR middle-end/101600 - Spurious -Warray-bounds downcasting a polymorphic pointer
PR middle-end/101977 - bogus -Warray-bounds on a negative index into a parameter in conditional with null

gcc/ChangeLog:

	PR middle-end/101600
	PR middle-end/101977
	* gimple-ssa-warn-access.cc (maybe_warn_for_bound): Tighten up
	the phrasing of a warning.
	(check_access): Use the remaining size after subtracting any offset
	rather than the whole object size.
	* pointer-query.cc (access_ref::get_ref): Clear BASE0 flag if it's
	clear for any nonnull PHI argument.
	(compute_objsize): Clear argument.

gcc/testsuite/ChangeLog:

	PR middle-end/101600
	PR middle-end/101977
	* g++.dg/pr100574.C: Prune out valid warning.
	* gcc.dg/pr20126.c: Same.
	* gcc.dg/Wstringop-overread.c: Adjust text of expected warnings.
	Add new instances.
	* gcc.dg/warn-strnlen-no-nul.c: Same.
	* g++.dg/warn/Warray-bounds-26.C: New test.
	* gcc.dg/Warray-bounds-88.c: New test.