Avoid checking control paths that will never be reached

[jdm]

When a loop will never terminate (ie. is conditioned on a value that is statically guaranteed to be true and the body contains no break statements), it would be nice not to throw errors about control paths following the loop that do not return a value.

[nikomatsakis]

Example:

while true {
   alt foo {
      case1 { ret ...; }
      case2 { ... }
   }
}

will currently complain because the function is not statically guaranteed to return (though of course it is (or else loop forever)).

[catamorphism]

Closing this because we have loop {...} now, and that should be used in these cases. If you see any limitations in that approach, please open a new bug.