LiveRegUnits: Rename accumulateBackward()->accumulate()

Contrary to the stepForward()/stepBackward() method accumulate() doesn't
have a direction as defs, uses and clobbers all have the same effect.

Also improve the documentation comment.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@307351 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed