value-based mark and sweep DCE (#14910)

value-based mark and sweep DCE (#14910)

Summary:
This makes DCE more granular by tracking live values/aliases through the graph (rather than just nodes). So we can be more aggressive in DCE around control flow blocks. For example, in:
```
%a0 = aten::foo()
%b = aten::foo()
%a2, %b2 = prim::If(%cond) {
  block0() {
    %a1 = aten::foo(%.0)
    %b1 = aten::foo(%b)
  } -> (%a1, %b1)
}
return (%a2)
```
we will now dce all the `%b` stuff.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/14910

Differential Revision: D13476445

Pulled By: suo

fbshipit-source-id: 2bf5db19711c07dde946697a4f4b270bd8baf791