[LoopUtils] Use llvm::find

Summary: Fixes this build error:

llvm/lib/Transforms/Utils/LoopUtils.cpp:679:26: error: no matching
function for call to 'find'
      Loop::iterator I = find(ParentLoop->begin(), ParentLoop->end(),
L);
                         ^~~~
Authored By: orivej
Reviewer: Whitney
Reviewed By: Whitney
Subscribers: hiraditya, llvm-commits
Tag: LLVM
Differential Revision: https://reviews.llvm.org/D80473