Simplifier: Re-do dependency analysis in abstractFloats (#24551)

In #24551, we abstracted a string literal binding over a type variable,
triggering a CoreLint error when that binding floated to top-level.

The solution implemented in this patch fixes this by re-doing dependency
analysis on a simplified recursive let binding that is about to be type
abstracted, in order to find the minimal set of type variables to abstract over.
See wrinkle (AB5) of Note [Floating and type abstraction] for more details.

Fixes #24551