Implement a todo, rewriting all possible scev expressions inside of the

Implement a todo, rewriting all possible scev expressions inside of the
loop.  This eliminates the extra add from the previous case, but it's
not clear that this will be a performance win overall.  Tommorows test
results will tell. :)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@13103 91177308-0d34-0410-b5e6-96231b3b80d8