Reapply r186316 with a fix for one bug where the code could walk off the

end of a vector. This was found with ASan. I've had one other report of
a crasher, but thus far been unable to reproduce the crash. It may well
be fixed with this version, and if not I'd like to get more information
from the build bots about what is happening.

See r186316 for the full commit log for the new implementation of the
SROA algorithm.

llvm-svn: 186565