[SCEV] Use some C++11'ism, NFC

Summary:

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D13457

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@249574 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed