teach SCEV that the scale and addition of an inbounds gep don't NSW.

This fixes a FIXME in scev-aa.ll (allowing a new no-alias result) and
generally makes things more precise.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@125449 91177308-0d34-0410-b5e6-96231b3b80d8