[LoopDeletion] (cleanup, NFC) Stop passing around reference to a vector

[LoopDeletion] (cleanup, NFC) Stop passing around reference to a vector
that we know has exactly one element when all we are going to do is get
that one element out of it.

Instead, pass around that one element.

There are more simplifications to come in this code...

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@292273 91177308-0d34-0410-b5e6-96231b3b80d8