[SCEV] Always sort AddRecExprs from different loops by dominance

Sorting of AddRecExprs by loop nesting does not make sense since we only invoke
the CompareSCEVComplexity for AddRecExprs that are used by one SCEV. This
guarantees that there is always a dominance relationship between them. This
patch removes the sorting by nesting which is a dead code in current usage of
this function.

Reviewed By: sanjoy

Differential Revision: https://reviews.llvm.org/D33228

llvm-svn: 303235