Review isNotExpression method for inlinings

[guillep]

          I don't understand completely this:

1. The method is negated (isNotExpression). It is more difficult to follow a negation than an affirmation. Would be nice to have an affirmative method and do the negation in the use: self isExpression not.
2. This definition has sense for all TParseNode? Or only for TReturnNode (who call this method).
3. I don't follow the condition anyway:

• Why if the paren is nil is not an expression?
• I understand that the only non-expression nodes are returns, assignments, (maybe if / case ?). Why is not asking that and is always asking for the parent?

Originally posted by @PalumboN in #836 (comment)