Minor cleanups & trivial refactoring related to Ast.

1) Literal::IsNull, IsTrue and IsFalse were dead code, and not needed.
2) No need to use the node type constants outside the Ast; there is IsSomeNodeType().
3) AsSomeNodeType() != NULL -> IsSomeNodeType().

R=rossberg@chromium.org
BUG=

Review URL: https://codereview.chromium.org/298143004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21621 ce2b1a6d-e550-0410-aec6-3dcde31c8c00