[LoopUnroll] Properly update loop-info when cloning prologues and epilogues.

Summary:
When cloning blocks for prologue/epilogue we need to replicate the loop
structure from the original loop. It wasn't a problem for the innermost
loops, but it led to an incorrect loop info when we unrolled a loop with
a child loop - in this case created prologue-loop had a child loop, but
loop info didn't reflect that.

This fixes PR28888.

Reviewers: chandlerc, sanjoy, hfinkel

Subscribers: llvm-commits, silvas

Differential Revision: https://reviews.llvm.org/D24203

llvm-svn: 280901