[C++11] Update to reflect the Value::use_iterator and
Value::user_iterator changes in LLVM r203364. Converts several of these
loops to nice range based loops in the process.

Built and tested cleanly for me, yay for being able to fully build and
test Polly changes!

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@203381 91177308-0d34-0410-b5e6-96231b3b80d8