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".  

llvm-svn: 373814