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

[via llvm-commits]

goldsteinn wrote:

Note, the new function `getScalarizationOfInsertElement` is quite similiar to `cheapToScalarize`. I couldn't really see a way to properly merge the two, but that might be a better direction.

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