Symbolically execute potential CRC loops and check whether the loop actually
calculates CRC (uses LFSR matching).  Calculated CRC and created LFSR are
compared on each iteration of the potential CRC loop.

gcc/

	* Makefile.in (OBJS): Add crc-verification.o.
	* crc-verification.cc: New file.
	* crc-verification.h: New file.
	* gimple-crc-optimization.cc (loop_calculates_crc): New function.
	(is_output_crc): Likewise.
	(swap_crc_and_data_if_needed): Likewise.
	(validate_crc_and_data): Likewise.
	(optimize_crc_loop): Likewise.
	(get_output_phi): Likewise.
	(execute): Add check whether potential CRC loop calculates CRC.
	* sym-exec/sym-exec-state.cc (create_reversed_lfsr): New function.
	(create_forward_lfsr): Likewise.
	(last_set_bit): Likewise.
	(create_lfsr): Likewise.
	* sym-exec/sym-exec-state.h (is_bit_vector): Reorder, make the function public and static.
	(create_reversed_lfsr) New static function declaration.
	(create_forward_lfsr) New static function declaration.