cmd/compile: mark concrete call of reflect.(*rtype).Method as REFLECTMETHOD
For functions that call reflect.Type.Method (or MethodByName), we
mark it as REFLECTMETHOD, which tells the linker that methods
can be retrieved via reflection and the linker keeps all exported
methods live. Currently, this marking expects exactly the
interface call reflect.Type.Method (or MethodByName). But now the
compiler can devirtualize that call to a concrete call
reflect.(*rtype).Method (or MethodByName), which is not handled
and causing the linker to discard methods too aggressively.
Handle the latter in this CL.