Set proper debug location for branch added in BasicBlock::splitBasicBlock().

Set proper debug location for branch added in BasicBlock::splitBasicBlock().

This improves debug locations in passes that do a lot of basic block
transformations. Important case is LoopUnroll pass, the test for correct
debug locations accompanies this change.

Test Plan: regression test suite

Reviewers: dblaikie, sanjoy

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D10367

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@239551 91177308-0d34-0410-b5e6-96231b3b80d8