Wraps more nicely.. it's weird to have the 0 on a different line from the
thing you're taking the channel of, versus reading left-to-right top-to-bottom
with nir_x you know you're interested in the x component.
Sometimes clearer, though this might be a wash.
However, they're sometimes more annoying than they're worth, so this MR adds the helpers as an optional convenience but doesn't convert the tree to use them.