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

[via llvm-commits]

tangaac wrote:

> 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.

Yes
It seems we should handle a vector that is `undef` or `poison` in a better away.
I will try in another PR.
 

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