Exploit slp-vectorization

[priyasiddharth]

We currently turn slp-vectorize off during seaopt

slp-vectorization turns the following pattern (from here)

while(1) {
  if(...) break;
  state->slots[index] = state->slots[next_index];
}

to

while(1) {
  if(...) break;
  state->slots[index] = state->slots[next_index];  // now pack into 2 element vector at index 0
  state->slots[index + 1] = state->slots[next_index + 1]; // pack at index 1
  // probably change counter stride width
}

so two scalar load/stores, become one vector/load store
Can we exploit slp-vectorization like this in some way?

Turned off in #454