[llvm-branch-commits] [llvm] [LoongArch] Broadcast repeated subsequence in build_vector instead of inserting per element (PR #154533)

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tangaac wrote:

We may do not need `XVREPLVE0`  and `XVREPLVE0Q`.
`XVREPLVE0` could be replaced to `vectorshuffle %v, poison, splat 0`  
`XVREPLVE0Q` could be replaced `xvpermi.q xr0, xr0, 2`

https://github.com/llvm/llvm-project/pull/154533