swift234

Fix double delete in generic specialization.

We ended up adding the same instruction twice to a SmallVector of
instructions to be deleted. To avoid this, we'll track these
to-be-deleted instructions in a SmallSetVector instead.

We were also failing to add an instruction that we can delete to the set
of instructions to be deleted, so I fixed that as well.

I've added a test case, but it's currently disabled because fixing this
turned up another issue in the same code which I still need to take a
look at.

Fixes rdar://problem/25369617.