We'll let the bytecode compiler and optimizing compilers deal with dead code,
rather than the ast visitors. The problem is that the visitors previously
disagreed upon what was dead. That's bad if necessary visitors omit parts of
the code that the bytecode generator will actually visit.

I did consider removing the AST nodes immediately in the parser, but that
adds overhead and actually broke code coverage. Since dead code shouldn't be
shipped to the browser anyway (and we can still omit it later in the bytecode
generator), I opted for keeping the nodes instead.

Change-Id: Ib02fa9031b17556d2e1d46af6648356486f8433d
Reviewed-on: https://chromium-review.googlesource.com/c/1470108
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59569}