[llvm] [MachinePipeliner] Remove UB from tests (NFC) (PR #123169)

[Ryotaro Kasuga via llvm-commits]

https://github.com/kasuga-fj commented:

I have a little time left, but for now I just want to fix the obvious (and maybe unintentional) UBs. The changes are mainly undef/null arguments used as pointer operands of a load/store. There are other tests that have the same UBs, but let me fix what was particularly unstable during the development of pipeliner.

https://github.com/llvm/llvm-project/pull/123169