[InstCombine] Fix poison propagation in round up alignment fold

We can't directly use the high bits value if it is more poisonous
due to poison elements in the masks.

This fixes the issue reported in
https://github.com/llvm/llvm-project/pull/88217#issuecomment-2061034941.

GitOrigin-RevId: 525d00e5edc4b83105c6ad518926b174f76c3a1c
2 files changed