[llvm] [LoongArch] lower SCALAR_TO_VECTOR to INSERT_VECTOR_ELT (PR #122863)

[Lu Weining via llvm-commits]

https://github.com/SixWeining commented:

Beyond this PR: If the insertion index is non-zero and the vector is `undef` or `poison`, stack store and load could also be replaced with vinsgr2vr.

https://github.com/llvm/llvm-project/pull/122863