[LoopUnroll] Use IRBuilder to create branch instructions.

Use IRBuilder::Create(Cond)?Br instead of constructing instructions
manually with BranchInst::Create(). It's consistent with other
uses of IRBuilder in this pass, and has an additional important
benefit:

Using IRBuilder will ensure that new branch instruction will get
the same debug location as original terminator instruction it will
eventually replace.

For now I'm not adding a testcase, as currently original terminator
instruction also lack debug location due to missing debug location
propagation in BasicBlock::splitBasicBlock. That is, the testcase
will accompany the fix for the latter I'm going to mail soon.

llvm-svn: 239550