fix function return type deduction implementation to conform language-spec

language spec allow either
```sushi
define fac(a: Int) =
    if a == 0: return 1
    return fac (a - 1)
```
or
```
define fac(a: Int) =
    if a > 0: return fac (a - 1)
    return 1
```
to successfully deducing `fac: Function Int Int` since one of the return statement is valid.
But currently implementation forces the return type declaration when function make recursive calls.