elide value cells for NLXs when it seems like the right thing
Previously we only did this for unsafe code.
Now we also elide them for
(1) exits from DX functions: if a DX function escapes its proper context,
trying to perform NLX to a stale tag is the least of our worries.
(2) functions that cannot escape. Since the escape analysis isn't yet very
tested, disable it for safe code, though. If this raises hairs on your
neck, consider this: even if our analysis is wrong, and a function we
didn't think could escape does, we're in the land of "undefined