beam_ssa_opt: Sink get_tuple_element before type optimization

Eagerly extracting elements can be quite problematic now that we
have union types. Consider the following:

0:
  %% _0 is {ok, #record{}} | {error,atom()}
  _1 = get_tuple_element _0, literal 0
  _2 = get_tuple_element _0, literal 1
  switch _1, label 0, [ { literal ok, label 1 }, { literal error, label 2 } ]
1:
  %% _0 is known to be {ok,#record{}} here
2:
  %% _0 is known to be {error,atom()} here

The type pass is clever enough to see that _0 has the types noted
above, but because the type of _2 is set in the first block where
we didn't have that information yet, it's still #record{} | atom()
in both branches.

Sinking these instructions to when they are used greatly improves
the type information in many cases, but it does have a few
limitations and using it in both of the above branches would take
us back to square one.

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