[llvm] [SelectionDAG] Fix unsafe cases for loop.dependence.{war/raw}.mask (PR #168565)

[Benjamin Maxwell via llvm-commits]

MacDue wrote:

Note: I'm taking over finishing this PR from Sam. I've rebased the changes (to fix some conflicts) and pushed some of my own fixups on top.

https://github.com/llvm/llvm-project/pull/168565