[pmo] Remove untested code around load_weak, store_weak, load_unowned…

[gottesmm]

…, store_unowned.

I am removing these for the following reasons:

• PMO does not have any tests for these code paths. (1).

• PMO does not try to promote these loads (it explicitly pattern matches load,
  copy_addr) or get available values from these (it explicitly pattern matches
  store or explodes a copy_addr to get the copy_addr's stores). This means that
  removing this code will not effect our constant propagation diagnostics. So,
  removing this untested code path at worst could cause us to no longer
  eliminate some dead objects that we otherwise would be able to eliminate at
  -Onone (low-priority). (2).

---

(1). I believe that the lack of PMO tests is due to this being a vestigal
remnant of DI code in PMO. My suspicion arises since:

 * The code was added when the two passes were both sharing the same use
   collector and auxillary data structures. Since then I have changed DI/PMO
   to each have their own copies.

 * DI has a bunch of tests that verify behavior around these instructions.

(2). I expect the number of actually removed allocations that are no longer
removed should be small since we do not promote loads from such allocations
and PMO will not eliminate an allocation that has any loads.

[gottesmm]

@swift-ci smoke test and merge