RFR: 8224861: Rework restrictions introduced by 8223029 for nullable arrays

[Roland Westrelin]

http://cr.openjdk.java.net/~roland/8224861/webrev.00/

JDK-8223029 introduced a number of restrictions to prevent C2 from
performing incorrect optimizations for nullable arrays of value
types. In some cases that logic is too conservative and is reworked in
this patch. With this change an array of nullable values is not exact
(because it has a subtype: array of non nullable values) even though its
element type is exact. The test change is to trigger a bug that was
caught when running with CompileThreshold=100. This change also includes
a Shenandoah build fix.

Roland.