foldAggregateConstructionIntoAggregateReuse iterates over the entries of
SourceAggregates and the order of inserted instructions depends on the
order of the iterator. Using a regular DenseMap can lead to
non-deterministic value naming/numbering.
I don't think it can actually impact the generated binary, but it makes
diffing IR more difficult.
PR: https://github.com/llvm/llvm-project/pull/132564
cfd53ffb44