[ARM][LowOverheadLoops] Fix loop count chain

Checking that the use-def chain that performs the loop count
isSafeToRemove is not sufficient because it means that we can
remove register copies that we need to restore lr to its correct
value. This change now prevents the transform from kicking in for the
'remove-elem-moves' test which needs to addressed later on.

Differential Revision: https://reviews.llvm.org/D74037