Request for reviews (S): 7143491: G1 C2 CTW: assert(p2x->outcnt() == 2) failed: expects 2 users: Xor and URShift nodes

[Vladimir Kozlov]

http://cr.openjdk.java.net/~kvn/7143491/webrev

7143491: G1 C2 CTW: assert(p2x->outcnt() == 2) failed: expects 2 users: Xor and 
URShift nodes

Constant Propagation or other ideal graph transformation may stored oop value 
withi NULL oop. As result part of G1 post barrier code (under the value check 
for NULL) become dead and removed.

Adjust the assert and code in eliminate_card_mark() method for such case. I also 
added the value transformation before generating store's barriers since it could 
be transformed to NULL and as results barriers will not be needed.

Verified with failed test.

Thanks,
Vladimir