fix: disambiguate '&' and '|' occurring within a returned expression

'&' and '|' are initially parsed into AmbiguousAmp and AmbiguousPipe. Later they
are discriminated into logical or bitwise operations. The traversal that did
this was not performing this disambiguation on the expression in a return
statement. As a result, returning something of the form '... & ...' or
'... | ...' would cause an error during code generation, even with a valid input
model.

While making this change, I realised there are further missing cases here. E.g.
it looks like shifts like '(... & ...) << ...' suffer from the same issue.

Github: related to #197 "& and | in function return expressions cause validation
  failures"