Correct error behavior of is_map_key/2 in guards

Consider the following functions:

    foo() -> bar(not_a_map).

    bar(M) when not is_map_key(a, M) -> ok;
    bar(_) -> error.

What will `foo/0` return? It depends. If the module is compiled
with the default compiler options, the return value will be
`ok`. If the module is compiled with the `inline` option,
the return value will be `error`.

The correct value is `error`, because the call to `is_map_key/2`
when the second argument is not a map should fail the entire
guard. That is the way other failing guards BIFs are handled.
For example:

    foo() -> bar(not_a_tuple).

    bar(T) when not element(1, T) -> ok;
    bar(_) -> error.

`foo/0` always returns `error` (whether the code is inlined
or not).

This bug can be fixed by changing the classification of `is_map_key/2`
in the `erl_internal` module. It is now classified as a type test,
which is incorrect because type tests should not fail. Reclassifying
it as a plain guard BIF corrects the bug.

This correction also fixes the internal consistency check
failure which was reported in:

https://bugs.erlang.org/browse/ERL-699

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