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
Florian Hahn