improve loop detection

[mflatt]

Generalize the recognized pattern from

 (letrec ([f (lambda ....)])
   (f arg ...))

to allow a loop entry into f to be anywhere in the letrec body as long as there's a single call to f as an entry point (and f is otherwise only tail-called within the lambda body). Also, avoid merging letrecs in the cpletrec pass, which can otherwise obscure loops.

Loops conversion by itself doesn't help all that much, since function calls are already fast. But loop conversion can enable floating-point unboxing, which can offer substantial performance benefits.