optimizer: convert `(let ([x M]) x)` to `(begin0 M #f)`
optimizer: convert `(let ([x M]) x)` to `(begin0 M #f)`

For simple enough M, `(let ([x M]) x)` is already converted
to just M, but add a conversion for other forms that gets rid
of the binding while preserving non-tailness.

mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket