C1 should fast-path Object.class.isInstance(obj) to null check

Current intrinsic code in C1 optimizes Class.isInstance (and Class.cast, transitively) quite well, but it misses an opportunity in optimizing for Object.class case.

Benchmark:
 http://cr.openjdk.java.net/~shade/8135033/ClassCastConstants.java

-XX:TieredStopAtLevel=1:

myClass_inlined 3.766 ± 0.021 ns/op
myClass_staticFinal 3.763 ± 0.002 ns/op
object_inlined 3.086 ± 0.001 ns/op
object_staticFinal 3.106 ± 0.038 ns/op

The disassembly for object_* tests show the redundant compare against Object.class.