ScalarEvolution: Construct SCEVDivision's Derived type instead of itself

SCEVDivision::divide constructed an object of SCEVDivision<Derived>
instead of Derived.  divide would call visit which would cast the
SCEVDivision<Derived> to type Derived.  As it happens,
SCEVDivision<Derived> and Derived currently have the same layout but
this is fragile and grounds for UB.

Instead, just construct Derived.  No functional change intended.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222126 91177308-0d34-0410-b5e6-96231b3b80d8