[Mem2Reg] Instantiate arbitrary empty types.
Currently, memory locations whose type is empty (`SILType::isEmpty`) are
regarded as viable sources for loads.
Previously, though, Mem2Reg only handled loads from empty types formed
only by tupling. Here, support is added for types formed also by
struct'ing. As before, this entails recursively instantiating the empty
types until reaching the innermost empty types (which aggregate nothing)
and then aggregating the resulting instances.
rdar://106224845
