Thanks for all the positive feedback!
I’ve made some preliminary PRs. As suggested by @dcaballe it doesn’t seem like this is going to be too much effort. PR [WIP][MLIR][Vector] Deprecate vector.splat by newling · Pull Request #147818 · llvm/llvm-project · GitHub (which is just for illustration) gets us most of the way there.
IREE folks: FWIW I intend to eventually make a similar PR in IREE where there are only a few uses of vector.splat