[Vectorizer]Make reductions-via-shifts and vec_shr_optab endianness-neutral

        * optabs.c (shift_amt_for_vec_perm_mask): Remove code conditional on
        BYTES_BIG_ENDIAN.
        * tree-vect-loop.c (calc_vec_perm_mask_for_shift,
        vect_create_epilog_for_reduction): Likewise.
        * doc/md.texi (vec_shr_m): Clarify direction of shifting.

From-SVN: r217550