Split the AST LoopStatement type into separate types for do/while,
while, and for loops.
Previously they were distinguished by a type field, which required
runtime asserts to avoid invalid nodes (since not all loop types have
the same internal structure). Now they C++ type system is used to
require well-formed loop ASTs.
Because they do not share compilation code, we had very large
functions in the code generators that merely did a runtime dispatch to
a specific implementation based on the type.
Review URL: http://codereview.chromium.org/269049
git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3048
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