erl_expand_records: Replace is_record() with matching

The compiler currently generates better code for:

 f(#r1{}) -> r1;
 f(#r2{}) -> r2;
 f(#r3{}) -> r3.

than for:

 g(X) when is_record(X, r1) -> r1;
 g(X) when is_record(X, r2) -> r2;
 g(X) when is_record(X, r3) -> r3.

The compiler generates good code for pattern matching (as in f/1),
but in g/1 there are no patterns to match, and the clause to be
executed must be chosen by evaluating the guards sequentially
until one succeeds.

Make the compiler generate better code by replacing calls to
is_record() with matching in the function head (basically,
g/1 will automatically be rewritten to do pattern matching
as in f/1).

Note that this rewrite will also benefit code such as:

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

because it would have been rewritten to:

 h(X) when (is_record(X, r1, 3) orelse fail) and (element(2, X) =:= 1) ->
     ok.

which in turn will be rewritten to:

 h({r1,_,_}=X) when (true orelse fail) and (element(2, X) =:= 1) ->
     ok.

(That will be further simplified in later compiler passes.)

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