LiveIntervalAnalysis: Cleanup handleMove{Down|Up}() functions, NFC

These two functions are hard to reason about. This commit makes the code
more comprehensible:

- Use four distinct variables (OldIdxIn, OldIdxOut, NewIdxIn, NewIdxOut)
  with a fixed value instead of a changing iterator I that points to
  different things during the function.
- Remove the early explanation before the function in favor of more
  detailed comments inside the function. Should have more/clearer comments now
  stating which conditions are tested and which invariants hold at
  different points in the functions.

The behaviour of the code was not changed.

I hope that this will make it easier to review the changes in
http://reviews.llvm.org/D9067 which I will adapt next.

Differential Revision: http://reviews.llvm.org/D16379

llvm-svn: 258756