Fix PR16651, an assert introduced in my recent re-work of the innards of
SROA.

The crux of the issue is that now we track uses of a partition of the
alloca in two places: the iterators over the partitioning uses and the
previously collected split uses vector. We weren't accounting for the
fact that the split uses might invalidate integer widening in ways other
than due to their width (in this case due to being volatile).

Further reduced testcase added to the tests.