[SCEV] Add one more case in computeConstantDifference

This patch teaches `computeConstantDifference` handle calculation of constant
difference between `(X + C1)` and `(X + C2)` which is `(C2 - C1)`.

Differential Revision: https://reviews.llvm.org/D43759
Reviewed By: anna

llvm-svn: 328609