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

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon commented:

I've taken a look - stretching everything across patches does it make it trickier, but stacked PRs aren't any better

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