IndVarSimplify: do not recompute an IV value outside of the loop if :

- it is trivially known to be used inside the loop in a way that can not be optimized away
- there is no use outside of the loop which can take advantage of the computation hoisting

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177432 91177308-0d34-0410-b5e6-96231b3b80d8