This change shaves off 15-20ns per iteration of the node.firstChild getter test in tests/html/bindings_perf.html. Based on [similar Gecko code](http://searchfox.org/mozilla-central/rev/f43c9e0ffa92e72dbdbcbf57eecf04a43d46da63/dom/bindings/BindingUtils.h#781). --- - [X] `./mach build -d` does not report any errors - [X] `./mach test-tidy` does not report any errors - [X] These changes fix (partially) #12358 Source-Repo: https://github.com/servo/servo Source-Revision: e2e7013e7629af1b1da4b944798bee00d4eae2d2