[llvm] [SDAG] Adding scalarization of `llvm.vector.insert` (PR #71614)

[via llvm-commits]

qcolombet wrote:

Thanks to https://github.com/llvm/llvm-project/pull/72048, I think we can abandon this one.

Technically the code that this PR touches is still broken without this PR IMHO, but we can't reach it anymore.

https://github.com/llvm/llvm-project/pull/71614