Current vectoriser doesn't support masked loads for SLP. We should add that, to
allow things like:

void
f (int *restrict x, int *restrict y, int *restrict z, int n)
{
  for (int i = 0; i < n; i += 2)
    {
      x[i] = y[i] ? z[i] : 1;
      x[i + 1] = y[i + 1] ? z[i + 1] : 2;
    }
}

to be vectorized using contiguous loads rather than LD2 and ST2.

This patch was motivated by SVE, but it is completely generic and should apply
to any architecture with masked loads.

From-SVN: r271704