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I had tried unrolling the loops to avoid requiring the `vector_store_elem` instruction, but it's arguably a problem to generate O(N) code for an operation on `@Vector(N, T)`. In addition, that lowering emitted a lot of `.aggregate_init` instructions, which is itself a quite difficult operation to codegen. This requires reintroducing runtime vector indexing internally. However, I've put it in a couple of instructions which are intended only for use by `Air.Legalize`, named `legalize_vec_elem_val` (like `array_elem_val`, but for indexing a vector with a runtime-known index) and `legalize_vec_store_elem` (like the old `vector_store_elem` instruction). These are explicitly documented as *not* being emitted by Sema, so need only be implemented by backends if they actually use an `Air.Legalize.Feature` which emits them (otherwise they can be marked as `unreachable`).