[NFC] Remove a minor compile-time optimization in Heap2Local

[kripken]

We tracked which expressions we saw an allocated struct/array reach, and then
quickly exited when another one did (as when two allocations mix, we can
optimize neither). It turns out that this helps very little in actual measurements
(looks like within noise - likely we are ruling out the un-optimizable cases early
otherwise anyhow). Also the complexity it adds is a problem for an improvement
I want to make to the pass, so remove it.