Constprop split_rest on tuple and namedtuple

[jakobnissen]

Julia 1.9 adds Base.split_rest, which can be used to slurp in non-final positions. However, unlike slurping in the final position, split_rest does not constant propagate, causing type instability in a pattern like head..., tail = args::Tuple.

This PR adds @constprop :aggressive to split_rest for tuples and named tuples, and tests that it constant folds as expected.

Issue discovered by @sgaure and fix proposed by @simeonschaub.
Closes #47326

[jakobnissen]

I'm wondering if a better solution is to add the @constprop :aggressive in the lowering step, since the arguments when lowering to split_rest are always known at compile time, and there might be gains from constant propagation across the board instead of for tuple and namedtuple only, but that fix is beyond me.

[simeonschaub]

Ah, beat me to it... 😄 This will have issues bootstrapping since @constprop isn't defined yet. Also, have you tested whether the annotation for the NamedTuple case is actually needed or whether the one for Tuple is already enough to fix this case too?

[jakobnissen]

It is sufficient to annotate tuple, and it will still infer for namedtuple. I'll close this PR - can you add tests for namedtuple in your PR then?