[llvm] [LV][NFC] Change undef to poison in most tests (PR #163525)

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

In some cases, the new changes make tests immediate UB or always result in poison.

Would be good to not use poison for most cases, 
* for phi nodes (better use a concrete start value)
* pointers for GEPs/loads/stores
* return values (those can just be changed to `ret void`)

https://github.com/llvm/llvm-project/pull/163525