Request for reviews (S): 6736417: Fastdebug C2 crashes in StoreBNode::Ideal

[Vladimir Kozlov]

http://webrev.invokedynamic.info/kvn/6736417/index.html

Fixed 6736417: Fastdebug C2 crashes in StoreBNode::Ideal

Problem:
Methods step_through_mergemem() and remove_dead_region() can cause
the node and its users to go dead but that isn't being checked for.

Solution:
- Check the result of step_through_mergemem() and remove_dead_region()
   methods and stop further transformations if the node become dead.
- Add the assert to transform_old() after the call to Ideal()
   to verify that it does not return a dead node.

Reviewed by:
Fix verified (y/n): y, failed test.

Other testing:
JPRT