IVUsers should only generate SCEV's for values that are safe to speculate.

This allows SCEVExpander to run on the IV expressions.

This codifies an assumption made by LSR to complete the fix for
PR11356, but I haven't been able to generate a separate unit test for
this part. I'm adding it as an extra safety check.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@160204 91177308-0d34-0410-b5e6-96231b3b80d8