Fix two bugs the LAllocator::FindOptimalSplitPos.

Fix two bugs the LAllocator::FindOptimalSplitPos.

- It was calculating start_block and end_block incorrectly.

- It was not considering the case when end_block is a loop header itself when searching for the header of the outermost loop.

These bugs do not affect correctness of the allocation but can severely degrade it's quality (cause spills in loop bodies).

Review URL: http://codereview.chromium.org/6901148

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7737 ce2b1a6d-e550-0410-aec6-3dcde31c8c00