[GlobalStatus] Keep Visited set in isSafeToDestroyConstant()

Constants cannot be cyclic, but they can be tree-like. Keep a
visited set to ensure we do not degenerate to exponential run-time.

This fixes the problem reported in https://reviews.llvm.org/D117223#3335482,
though I haven't been able to construct a concise test case for
the issue. This requires a combination of dead constants and the
kind of constant expression tree that textual IR cannot represent
(because the textual representation, unlike the in-memory
representation, is also exponential in size).