c++: ICE with -Wtautological-compare in template [PR116534]

c++: ICE with -Wtautological-compare in template [PR116534]

Pre r14-4793, we'd call warn_tautological_cmp -> operand_equal_p
with operands wrapped in NON_DEPENDENT_EXPR, which works, since
o_e_p bails for codes it doesn't know.  But now we pass operands
not encapsulated in NON_DEPENDENT_EXPR, and crash, because the
template tree for &a[x] has null DECL_FIELD_OFFSET.

This patch extends r12-7797 to cover the case when DECL_FIELD_OFFSET
is null.

	PR c++/116534

gcc/ChangeLog:

	* fold-const.cc (operand_compare::operand_equal_p): If either
	field's DECL_FIELD_OFFSET is null, compare the fields with ==.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/Wtautological-compare4.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>