[SROA] Update the documentation and names for accessing the slices

[SROA] Update the documentation and names for accessing the slices
within a partition of an alloca in SROA.

This reflects the fact that the organization of the slices isn't really
ideal for analysis, but is the naive way in which the slices are
available while we're processing them in the core partitioning
algorithm.

It is possible we could improve matters, and I've left a FIXME with
one of my ideas for how to do this, but it is a lot of work, the benefit
is somewhat minor, and it isn't clear that it would be strictly better.
=/ Not really satisfying, but I'm out of really good ideas.

This also improves one place where the debug logging failed to mark some
split partitions. Now we log in one place, slightly later, and with
accurate information about whether the slice is split by the partition
being rewritten.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224800 91177308-0d34-0410-b5e6-96231b3b80d8