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

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

After #145939 I tried to create a first step to help with this on #146332 but to address regressions its starting to look at lot like this patch tbh - please can you rebase and help identify the missing opcodes we need to add to the canCreateUndefOrPoison callbacks?

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