RFR(XS): 8214728: Unnecessary InstanceKlass::cast at few places

[Calvin Cheung]

bug: https://bugs.openjdk.java.net/browse/JDK-8214728
webrev: http://cr.openjdk.java.net/~ccheung/8214728/webrev.00/

A simple change to remove unnecessary InstanceKlass::cast at 3 places.

Ran hs-tier{1,2,3} tests.

thanks,
Calvin