RFR: 8238190 : [JVMCI] Fix single implementor speculation for diamond shapes.

[David Leopoldseder]

Hi,

Please review this tiny change (+ a new test case exercising the pattern) to
fix the single implementor logic in instanceKlass for diamond shaped 
class hierarchies.

This bug was revealed in Graal where we use the single implementor logic 
for invokeinterface devirtualization.

https://bugs.openjdk.java.net/browse/JDK-8238190
http://cr.openjdk.java.net/~davleopo/JDK-8238190/webrev.001/

Thank you.

- david