Coccinelle:

  @@
  expression a, b;
  @@
  -a ? a : b
  +a ?: b

Applied with:

  spatch --sp-file ternary.cocci --in-place --smpl-spacing --dir .

With some manual adjustments on spots that Cocci didn't catch for
reasons unknown.

Thanks to the marvelous effort of the GNU compiler developer we can now
spare a couple of bits that could be used for more important things,
like this commit message. Standards commitees yet have to catch up.