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

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

Similarly - should we be allowed to remove freezes from constant pool loads entirely (maybe with some handling for undef elts in the Constant)?

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