[RegionInfo] Remove unused and broken function splitBlock

Summary:
It always makes NewBB the entry of the region instead of OldBB. This breaks if there are edges from inside the region to OldBB. OldBB is moved out of the region and hence there are exiting edges to OldBB and the region's exit block, contradicting the single-exit condition for regions.

The only use from Polly is going to be removed, hence I propose to remove the function completely.

Reviewers: grosser

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D11873

llvm-svn: 245092