Eagerly resolve sum nodes during parsing
Product nodes are eagerly resolved during parse time, but sum nodes are
not. This might cause floating point inprecision in sum nodes, which
leads to invalid calculations, e.g. round(down, (7 - 1) / 3, 1) would
end up being round(down, (2.3333333 - 0.33333334), 1), then
round(down, 1.99999996, 1), which equals 1, which is incorrect.
Differential Revision: https://phabricator.services.mozilla.com/D225936
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1924363
gecko-commit: 2943453155ac4ea275b37a12d30391c39f1a3c1e
gecko-reviewers: layout-reviewers, emilio