Evaluate element/2 at compile-time using type information

The erl_expand_records compiler pass translates the
following code:

  h(X) when X#r1.a =:= 1 -> ok.

to (essentially):

  h({r1,V1,V2}=X) when element(2, X) =:= 1 -> ok.

Since the guard can only be executed when the pattern matching
has succeeded, we know that the second element in the tuple X
must have been bound to V2. Thus we can eliminate the call
to element/2 like this:

  h({r1,V1,V2}=X) when V1 =:= 1 -> ok.

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