DmdAnal: LetUp doesn't `keepAliveDmdEnv` RULE FVs

In the fix for #18638 (closed) (and possibly #11126 (closed)), we keepAliveDmdEnv RULE FVs in dmdAnalRhsLetDown.

We should also do so in the LetUp case, where we currently don't. I'm not sure in which scenarios that matters.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information