[llvm] [SelectionDAG] Improve isGuaranteedNotToBeUndefOrPoison (PR #147019)

[Björn Pettersson via llvm-commits]

bjope wrote:

> Can you seperate off the peekThroughInsertVectorElt change into a followup PR? I think that's the only thing causing hesitation on this patch - the subvector handling in particular looks trivial.

Sure. I rebased and updated the patch to skip the peek through at this stage.

https://github.com/llvm/llvm-project/pull/147019