Update phis in AMDGPUUnifyDivergentExitNodes

Original patch https://reviews.llvm.org/D63659 from
Steven Perron <stevenperron@google.com>

The pass AMDGPUUnifyDivergentExitNodes does not update the phi nodes in
the successors of blocks that is splits. This is fixed by calling
BasicBlock::splitBasicBlock to split the block instead of doing it
manually. This does extra work because a new conditional branch is
created in BB which is immediately replaced, but I think the simplicity
is worth it. It also helps make the code more future proof in case other
things need to be updated.

llvm-svn: 364342