[RISCV] Pass structs containing riscv_rvv_vector_bits types using vector registers. (#203601) The ABI for this isn't documented. The riscv_rvv_vector_bits feature was copied from aarch_sve_vector_bits which passes the types in vector registers. At the time I implemented RISC-V I didn't think about struct arguments so no ABI support was added. I think it makes sense to pass structs of these types in vector registers similar to the vls_cc ABI. We already do use vector registers for these types when the vls_cc ABI is enabled for a function(unclear if that was intentional), but we should do it unconditionally. I will work with gcc maintainers to see if they can do the same. In practice, there probably isn't much compiler mixing with these types. This patch was heavily assisted by AI, but I've reviewed it and the changes seem reasonable to me.
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