Minor cleanups & trivial refactoring related to Ast.

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=

Committed: https://code.google.com/p/v8/source/detail?r=21621

[marja, 2014-05-30 16:12:57]

rossberg, ptal. Just some minor cleanups which I carved out of the symbol table
CL.

[rossberg, 2014-06-02 12:57:58]

lgtm

[marja, 2014-06-03 07:41:12]

Committed patchset #2 manually as r21621 (presubmit successful).