testsuite: Fix vect/vect-sdiv-pow2-1.c

testsuite: Fix vect/vect-sdiv-pow2-1.c

We're now able to vectorise the set-up loop:

      int p = power2 (fns[i].po2);
      for (int j = 0; j < N; j++)
        a[j] = ((p << 4) * j) / (N - 1) - (p << 5);

This patch adds an asm to stop the loop being vectorised.

gcc/testsuite/
	* gcc.dg/vect/vect-sdiv-pow2-1.c (main): Add an asm to the
	set-up loop.