Expressions of the form

    a_0 + a_1 + a_2 + a_3 + ... + a_n

seem to be reasonably common for cases such as building templates.
However, parsing these expressions results in a n-deep expression tree:

           ...
          /
         +
        / \
       +  a_2
      / \
    a_0 a_1

Traversing this tree during compilation can cause a stack overflow when n is
large.

Instead, for left-associate operations such as add, we now build up an
n-ary node in the parse tree, of the form

         n-ary +
       /  |      \
      /   |  ...  \
    a_0  a_1      a_n

The bytecode compiler can now iterate through the child expressions
rather than recursing.

This patch only supports arithmetic operations -- subsequent patches
will enable the same optimization for logical tests and comma
expressions.

Bug: v8:6964
Bug: chromium:724961
Bug: chromium:731861
Bug: chromium:752081
Bug: chromium:771653
Bug: chromium:777302
Change-Id: Ie97e4ce42506fe62a7bc4ffbdaa90a9f698352cb
Reviewed-on: https://chromium-review.googlesource.com/733120
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48920}