[rs6000] Fix PR92566 by checking VECTOR_UNIT_NONE_P

[rs6000] Fix PR92566 by checking VECTOR_UNIT_NONE_P

As Segher pointed out in PR92566, we shouldn't offer some vector modes which
aren't supported under current setting.  This patch is to make it check by
VECTOR_UNIT_NONE_P which is initialized as current architecture masks.

2019-11-28  Kewen Lin  <linkw@gcc.gnu.org>

    PR target/92566
    * gcc/config/rs6000/rs6000.c (rs6000_preferred_simd_mode): Check by
    VECTOR_UNIT_NONE_P instead.

From-SVN: r278800