[SCEV] Fix PR36974.

The patch changes the usage of dominate to properlyDominate
to satisfy the condition !(a < a) while using std::max.

It is actually NFC due to set data structure is used to keep
the Loops and no two identical loops can be in collection.
So in reality there is no difference between usage of
dominate and properlyDominate in this particular case.
However it might be changed so it is better to fix it.

llvm-svn: 329051