Fix two bugs in LiveRange::SplitAt:
Fix two bugs in LiveRange::SplitAt:

- when splitting at the beginning of the UseInterval we need to find an interval preceding it.

- we need to reset cached iteration state after splitting.

BUG=v8:1410

Review URL: http://codereview.chromium.org/7053039

git-svn-id: b158db1e4b4ab85d4c9e510fdef4b1e8c614b15b@8123 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh