[testsuite] Fix PR92464 by adjust test case loop bound

[testsuite] Fix PR92464 by adjust test case loop bound

  The recent vectorization cost adjustment on load leads
  the profitable min iteration count to change from 19 to 12.
  The case happens to hit the threshold.  This patch is to
  adjust the loop bound from 16 to 14.

  gcc/testsuite/ChangeLog

  2019-11-14  Kewen Lin  <linkw@gcc.gnu.org>

    PR target/92464
    * gcc.dg/vect/costmodel/ppc/costmodel-vect-76b.c: Adjust
    loop bound due to load cost adjustment.

From-SVN: r278195