RemoveRedundantTypeCast: keep casts on signature-polymorphic methods

[timtebeek]

• Fixes RemoveRedundantTypeCast removes the cast that gives a MethodHandle#invoke call its return type #1024

MethodHandle#invoke / invokeExact and the VarHandle accessors are signature-polymorphic (JLS 15.12.3): the call site has no fixed signature, and the enclosing cast is what supplies the return type. The recipe compared the attributed expression type against the cast type, but that attributed type only exists because of the cast, so the check always succeeded and the cast was always removed — leaving code that no longer compiles:

error: incompatible types: Object cannot be converted to String
        return handle.invoke();

Now a cast whose expression is a method invocation declared on java.lang.invoke.MethodHandle or java.lang.invoke.VarHandle is preserved.