Generalize optimization of "one-armed" cases

[bjorng]

In general, a 'case' expression will force a stack frame.

However, for an one-armed 'case':

  case Expr of
     Pat1 ->
       DoSomething;
     Pat2 ->
       erlang:error(bad)
  end.

at the end of a function there is no need to allocate a
stack frame

In sys_core_fold there is an optimization that takes advantage
of this by rewriting code such as:

  case Expr of
     Pat1 ->
       DoSomething;
     Pat2 ->
       erlang:error(bad)
  end,
  MoreCode.

to:

  case Expr of
     Pat1 ->
       DoSomething,
       MoreCode;
     Pat2 ->
       erlang:error(bad)
  end.

However, that optimization would not be applied if there
were more than one failing clause such as in this code:

  case Expr of
     Pat1 ->
       DoSomething,
       MoreCode;
     Pat2 ->
       erlang:error(bad);
     _ ->
       erlang:error(case_clause)
  end.

Generalize the optimization to handle any number of failing
clauses at the end of the case.

Reported-by: bugs.erlang.org/browse/ERL-452