[llvm] [InstCombine] Scalarize `(vec_ops (insert ?, X, Idx))` when only one element is demanded (PR #84645)

[via llvm-commits]

goldsteinn wrote:

@dtcxzyw can you test this. IMO its not worth the extra complexity unless it shows up in realworld.

https://github.com/llvm/llvm-project/pull/84645