Better reachability analysis

[JelleZijlstra]

For these functions:

def f() -> int:
    while True:
          return 1

def g() -> int:
     for x in it:
          if x:
               return x
     else:
          return 1

pyanalyze will currently complain about a missing return value. Instead we should detect that code after the loop is unreachable.

Ideally we should also give an error for unreachable code that is not "assert False".