[SCEV] Use trivial bound on defining scope of all SCEVs when computing flags

[SCEV] Use trivial bound on defining scope of all SCEVs when computing flags

This addresses a comment from review on D109845.  Even for SCEVs which we can't find true bounds without recursing through operands, entry to the function forms a trivial upper bound.  In some cases, this trivial bound is enough to prove safety of flag inference.