RFR [9, 10]: Incorrect cast to not-null in LibraryCallKit::inline_array_equals

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/c2-array-equals/webrev.01/

This is the regression caused by latest fix in String intrinsics. The change was incorrect: the
intrinsic does accept null arguments, and casting them to non-null messes things up. gcbench seems
to hit this bug every time, which manifests as the crash in C2-compiled code.

FYI: I am going to push the variant of this fix to sh/jdk9 too.

Testing: hotspot_gc_shenandoah, failing gc-bench tests

Thanks,
-Aleksey