This function doesn't seem to be used on performance critical paths. Making it inline causes many extra headers to be included. E.g., we can remove the dependency on the large header classfile/javaClasses.hpp.

open/src/hotspot$ whoincludes instanceKlass.inline.hpp
scanning 521 instanceKlass.inline.hpp
   2 found 521 iterator.inline.hpp
   3 found 521 instanceMirrorKlass.inline.hpp
   4 found 521 instanceClassLoaderKlass.inline.hpp
   5 found 521 instanceRefKlass.inline.hpp
   6 found 521 javaClasses.inline.hpp

(See https://github.com/iklam/tools/blob/main/headers/whoincludes.tcl )

(TBD: need to do perf measurements)