This PR fixes the `floatLetIn` pass to not move variables in case it
could break linearity (owned variables being passed with RC 1). This
mostly improves the situation in the parser which previously had many
functions that were supposed to be linear in terms of `ParserState` but
the compiler made them non-linear. For an example of how this affected
parsers:
```lean-4
def optionalFn (p : ParserFn) : ParserFn := fun c s =>
let iniSz := s.stackSize
let iniPos := s.pos
let s := p c s
let s := if s.hasError && s.pos == iniPos then s.restore iniSz iniPos else s
s.mkNode nullKind iniSz
```
previously moved the `let iniSz := ...` declaration into the `hasError`
branch. However, this means that at the point of calling the inner
parser (`p c s`), the original state `s` needs to have RC>1 because it
is used later in the `hasError` branch, breaking linearity. This fix
prevents such moves, keeping `iniSz` before the `p c s` call.
