Current vectoriser doesn't support masked loads for SLP.

Current vectoriser doesn't support masked loads for SLP.

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