Fix a *nasty* miscompile in experimental unordered atomic lowering
This is an omission in rL371441. Loads which happened to be unordered weren't being added to the PendingLoad set, and thus weren't be ordered w/respect to side effects which followed before the end of the block.
Included test case is how I spotted this. We had an atomic load being folded into a using instruction after a fence that load was supposed to be ordered with. I'm sure it showed up a bunch of other ways as well.
Spotted via manual inspecting of assembly differences in a corpus w/and w/o the new experimental mode. Finding this with testing would have been "unpleasant".