[MachinePipeliner] Remove UB from tests (NFC) (#123169)
This patch removes UB from some tests for MachinePipeliner. This patch
fixes following cases.
- Branching on an `undef` value.
- Using `undef`/`null` as a pointer operand of a load/store.
There are other tests of pipeliner that contain the same UB, but for
now, this patch fixes particularly unstable cases when I developed
pipeliner.
