[SROA] Start more deeply moving SROA to use ranges rather than just

iterators.

There are a ton of places where it essentially wants ranges
rather than just iterators. This is just the first step that adds the
core slice range typedefs and uses them in a couple of places. I still
have to explicitly construct them because they've not been punched
throughout the entire set of code. More range-based cleanups incoming.

llvm-svn: 219955