[Mem2Reg] Undef types in empty projection sources.
Mem2Reg may materialize the unique instance of empty types when
promoting an address of that empty type. Previously, it was required
that the top-most type in the aggregate be empty. This failed to handle
the case where a projection of empty type from a non-empty aggregate was
promoted.
For example:
```
%addr = alloc_stack $((), C)
%empty_addr = tuple_element_addr $addr : $*((), C), 0
%addr = load %empty_addr : $*()
```
where `C` is some non-empty type.
Here, that case is handled by using `undef` for each non-empty field in
the projected-from aggregate.
In the example,
```
%empty = tuple ()
%tuple = tuple (%empty : $(), undef : $C)
%empty_again = tuple_extract %tuple : $((), C), 0
```
rdar://122417297
