Generalize optimization of "one-armed" cases

A 'case' expression will force a stack frame (essentially in the same
way as a function call), unless it is at the end of a function.

In sys_core_fold there is an optimization that can optimize one-armed
cases such as:

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

Because only one arm of the 'case' can succeed, the code after the
case can be move into the successful arm:

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

Thus, the 'case' is at the end of the function and it will no longer
need a stack frame.

However, the optimization in sys_core_fold 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

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