Add a utility function for mirror to InstanceKlass

There are many instances in the JVM where we have a mirror and want the InstanceKlass pointer instead.

Doing this access inline seems to help microbenchmarks a little bit for the callers of static InstanceKlass::allocate_instance(java_mirror, TRAPS).

The utility function can be inlined in instanceKlass.inline.hpp (or klass.inline.hpp ?) and used in several other places as a cleanup.

See discussion in:
https://github.com/openjdk/jdk/pull/12782