[LoopUnroll] Use LoopSize+1 as threshold, to allow unrolling loops matching LoopSize.

We use `< UP.Threshold` later on, so we should use LoopSize + 1, to
allow unrolling if the result won't exceed to loop size.

Fixes PR43305.

Reviewers: efriedma, dmgreen, paquette

Reviewed By: dmgreen

Differential Revision: https://reviews.llvm.org/D67594

llvm-svn: 372084