fold-const: Handle C++ dependent COMPONENT_REFs in operand_equal_p [PR105035]

As mentioned in the PR, operand_equal_p already contains some hacks so that
it can be called already on pre-instantiation C++ trees from templates,
but the recent change to compare DECL_FIELD_OFFSET in the COMPONENT_REF
case broke this.  Many such COMPONENT_REFs are already punted on earlier
because they have NULL TREE_TYPE, but in this case the code knows what
type they have but still uses an IDENTIFIER_NODE as second operand
of COMPONENT_REF (I think SCOPE_REF is something that could be used too).

The following patch looks at those DECL_FIELD_*OFFSET fields only if
both field[01] args are FIELD_DECLs and otherwise keeps it to the
earlier OP_SAME (1) check that guards this whole block.

2022-03-24  Jakub Jelinek  <jakub@redhat.com>

	PR c++/105035
	* fold-const.cc (operand_equal_p) <case COMPONENT_REF>: If either
	field0 or field1 is not a FIELD_DECL, return false.

	* g++.dg/warn/Wduplicated-cond2.C: New test.