Handle constant CONSTRUCTORs in operand_compare

Handle constant CONSTRUCTORs in operand_compare

This teaches operand_compare to compare constant CONSTRUCTORs, which is
quite helpful for so-called fat pointers in Ada, i.e. objects that are
semantically pointers but are represented by structures made up of two
pointers.  This is modeled on the implementation present in the ICF pass.

gcc/
	* fold-const.cc (operand_compare::operand_equal_p) <CONSTRUCTOR>:
	Deal with nonempty constant CONSTRUCTORs.
	(operand_compare::hash_operand) <CONSTRUCTOR>: Hash DECL_FIELD_OFFSET
	and DECL_FIELD_BIT_OFFSET for FIELD_DECLs.

gcc/testsuite/
	* gnat.dg/opt103.ads, gnat.dg/opt103.adb: New test.