Rename LiveIntervalAnalysis.h to LiveIntervals.h

Headers/Implementation files should be named after the class they
declare/define.

Also eliminated an `#include "llvm/CodeGen/LiveIntervalAnalysis.h"` in
favor of `class LiveIntarvals;`

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@320546 91177308-0d34-0410-b5e6-96231b3b80d8