[SCEV] Change the interface of computeConstantDifference; NFC

This is in preparation of
s/getConstantDifference/computeConstantDifference/ in a later change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@276503 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed