Remove unused parameter 'phase' in AllocateNode::Ideal_allocation

There are 2 overloaded AllocateNode::Idea_allocation() in graphkit.cpp.
One of them never uses 'phase' in the pattern-matching effort.

AllocateNode::Ideal_allocation(Node* ptr, PhaseValues* phase) {
...
}

C++ compiler may emit a warning for the unused parameter. We will need to take care of it if we treat warning as error. It also unnecessarily couple CheckCastPP with PhaseValue. In some places, we have to gain the instance for it.I would like to remove it. This is a pure clean-up.

The other Idea_allocation() does use PhaseValue* to get constant nodes to get constant nodes, so leave it alone.