[llvm] [SelectionDAG] Deal with POISON for INSERT_VECTOR_ELT/INSERT_SUBVECTOR (part 1) (PR #143102)

[Björn Pettersson via llvm-commits]

bjope wrote:

I'm still looking for advice on how to move forward with this.

- Add more reviewers?
- Re-order patches to avoid temporary test regressions (i.e. do fixups first)?  (problem here is I would need to find motivating test cases)?
- Squash everything into a mega patch?
- Split up in even smaller patches?
- Do I need to get all patches (including #143103 and #143105) approved and landed simultaneously?


As I see it fixing #141034 is needed to move forward with the fixes for #138513 (as they rely on that insert of UNDEF isn't surprisingly turned into inserting POISON). I've been stalling on updating the patches for #138513 while waiting for this patch to land.

https://github.com/llvm/llvm-project/pull/143102