commit ec5d94291 made is to that the nocode_wanted state from
inside a statement expression is retained after it. That is wrong
if the statement expression can't be entered to start with. In the
latter case the state from before the stmt-expr is the one we need.
If a condition is always zero/non-zero we can omit the
then or else code. This is complicated a bit by having to
deal with labels that might make such code reachable without
us yet knowing during parsing.
