Use a scalar pointer when emitting IR for a scalar bucket

[drhagen]

Right now, a point-like bucket, looks like this:

double* restrict bucket_y_0 = y_vals + p_y_0_0;
int32_t i_bucket_y_0 = 0;
while (i_bucket_y_0 < 1) {
  bucket_y_0[i_bucket_y_0] = 0;
  i_bucket_y_0++;
}

// *** Computation of expression ***
bucket_y_0[0] += A_vals[p_A_1_1] * x_vals[p_x_2_0];

This is not wrong, and it is probably all optimized down by LLVM, but it would be fairly simple to emit this instead, which looks a lot nicer:

double* restrict bucket_y_0 = y_vals + p_y_0_0
*bucket_y_0 = 0

// *** Computation of expression ***
*bucket_y_0 += A_vals[p_A_1_1] * x_vals[p_x_2_0];