buffer: improve allocation performance

assertSize() is adjusted to be inlineable according to V8's default
function size limits when determining inlineability. This results in
up to 11% performance gains when allocating any kind of Buffer.

Avoid avoids use of in, resulting in ~50% improvement when creating
a Buffer from an array-like object.

PR-URL: https://github.com/nodejs/node/pull/10443


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>