[vector] RFR (S): CheckCastPP hinders vector box elimination

[Vladimir Ivanov]

http://cr.openjdk.java.net/~vlivanov/panama/vector/8221634.checkcastpp/webrev.00/
https://bugs.openjdk.java.net/browse/JDK-8221634

CheckCastPP nodes may hinder vector box elimination.

Incremental inlining, in particular, produces "CheckCastPP (VectoBox 
...)" shape when a vector is returned from a method and CheckCastPP node 
stays there if it's an upcast to Vector/XxxVector from XxxNnnVector, 
thus blocking VectorBox from being eliminated.

The fix is to add a special case into CheckCastPPNode::Identity() to 
optimize such particular case with VectorBox.

Best regards,
Vladimir Ivanov