Move misplaced assignment in num_sign_bit_copies1

The old assignment to bitwidth was before we handled VOIDmode with:

      if (mode == VOIDmode)
        mode = GET_MODE (x);

so when VOIDmode was specified we would always use:

      if (bitwidth < GET_MODE_PRECISION (GET_MODE (x)))
        {
          num0 = cached_num_sign_bit_copies (x, GET_MODE (x),
                                             known_x, known_mode, known_ret);
          return MAX (1,
                      num0 - (int) (GET_MODE_PRECISION (GET_MODE (x)) - bitwidth));
        }

For a zero bitwidth this always returns 1 (which is the most
pessimistic result).

gcc/
2016-11-15  Richard Sandiford  <richard.sandiford@arm.com>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

	* rtlanal.c (num_sign_bit_copies1): Calculate bitwidth after
	handling VOIDmode.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>

From-SVN: r242440