Simplifier: Inlining trivial let can lose sharing

While working on #10613, I looked a bit more closely at the ticky numbers of single-entry thunks. In all of nofib, I found exactly one fishy result: A thunk, marked as single entry, but entered multiple times, in gametb.

I’ll write a preliminary analysis in the comments and either found a bug or a misunderstanding on my side.