When GopherJS flattens a loop into a while-switch-case construct, it
needs to generate an instruction to go back to the beginning of the loop
after the last statement of the loop body. However, when the last
statement is a control statement itself (return, continue, break, goto),
this instruction becomes unnecessary and unreachable. Example Go code:

```go
func foo() {
  for {
    // Do something
    break
  }
}
```

This change prevents such an instruction from being generated. This has
a marginal artifact size reduction (1632 bytes on todomvc), but also
eliminates a warning Firefox prints in the console, which may be
confusing to users (gopherjs#1069, gopherjs#575).

The improved check is able to detect cases when the return statement is
wrapped in another statement, but is effectively the last executed
statement in a list, for example:

```go
func withGoto() (resultType, error) {
  // Do something.
  return result, nil
Error:
  return nil, err // This is inside an *ast.LabeledStmt!
}

func withBlock() resultType {
  // Do something.
  {
    // Do more.
    return result // This is inside a *ast.BlockStmt!
  }
}
```

Better detection prevents generation of unnecessary return statements at
the end of a function (marginally smaller output size), as well as
prevents "unreachable code" warnings in Firefox (gopherjs#575, gopherjs#1069).