[RISCV] Handle zeroinitializer of vector tuple Type (#113995)

It doesn't make sense to add a new generic ISD to handle riscv tuple
type. Instead we use `SPLAT_VECTOR` for ISD and further lower to

[RISCV] Handle zeroinitializer of vector tuple Type (#113995)

It doesn't make sense to add a new generic ISD to handle riscv tuple
type. Instead we use `SPLAT_VECTOR` for ISD and further lower to
`VMV_V_X`.

Note: If there's `visitSPLAT_VECTOR` in generic DAG combiner, it needs
to skip riscv vector tuple type.

Stack on https://github.com/llvm/llvm-project/pull/114329

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