VM: Reduce size of AST-nodes vtables.
VM: Reduce size of AST-nodes vtables.

Make Is## methods of AST nodes non-virtual, like we do for IL instruction nodes.

This saves around 10k in vtables on ia32.

BUG=
R=rmacnak@google.com

Review URL: https://codereview.chromium.org/1482153006 .

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