(CONST_OK_FOR_LETTER_P): Change meaning of 'K'.

(CONST_OK_FOR_LETTER_P): Change meaning of 'K'.  Add new
alternative 'N'.  Use VAL_*_BITS_P macros when possible.
(LEGITIMATE_CONSTANT_P) Fix comment.
(FITS_14_BITS): Remove.
(SMALL_INT): Remove.

From-SVN: r3241