partially implement RSyntaxNodeSPI.getRequalsImpl

...rc/com/oracle/truffle/r/nodes/builtin/base/Identical.java

...s/src/com/oracle/truffle/r/nodes/access/ConstantNode.java

...le/truffle/r/nodes/access/variables/ReadVariableNode.java

...des/src/com/oracle/truffle/r/nodes/control/BlockNode.java

...com/oracle/truffle/r/nodes/function/FunctionBodyNode.java

...acle/truffle/r/nodes/function/FunctionDefinitionNode.java

...om/oracle/truffle/r/nodes/function/GroupDispatchNode.java

...es/src/com/oracle/truffle/r/nodes/function/RCallNode.java

...e/src/com/oracle/truffle/r/runtime/RRuntimeASTAccess.java