Fix RakudoScalar as having non-invoking fetch.
Optimization that avoids a Rakudo Moar bug.

Doing $some_int == Inf ends up coercing $some_int and re-dispatching,
which we can avoid much more quickly with a type check for the very
common case of passing an integer to gimme. This helps all backends,
but also avoids a Moar bug; hopefully sanity tests will show it up in
a clearer way.