Teach case_opt/3 to avoid unnecessary building

Given this code:

  f(S) ->
    F0 = F1 = {S,S},
    [F0,F1].

case_opt/3 would "optimize" it like this:

  f(S) ->
    F1 = {S,S},
    F0 = {S,S},
    [F0,F1].

Similarly, this code:

  g({a,_,_}=T) ->
    {b,
     [_,_] = [T,none],
     x}.

would be rewritten to:

  g({a,Tmp1,Tmp2}=T) ->
    Tmp3 = {a,Tmp1,Tmp2},
    {b,
     [Tmp3,none],
     x}.

where the tuple is rebuilt instead of using the T variable.

Rewrite case_opt/3 to be more careful while optimizing.

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