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

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

Are you saying that SLP is creating these vectorizations but shouldn't? Isnt that a cost issue with the alt opcode cases?

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