RFR(XS): 8146999: hotspot/test/compiler/c2/8007294/Test8007294.java test nightly failure

[Roland Westrelin]

> http://cr.openjdk.java.net/~roland/8146999/webrev.00/

Further testing revealed this bug was hiding another one.

http://cr.openjdk.java.net/~roland/8146999/webrev.01/

In PhiNode::unique_input(), uncast() could step over the CheckCastPP node that follows an Allocation. If that happens, a new CheckCastPP will be created and will replace the CheckCastPP of the allocation but with a different control and it could cause a safepoint to be in between the allocation and the CheckCastPP and assert failures with "there should be a oop in OopMap instead of a live raw oop at safepoint”.

Roland.