[llvm] [PredicateInfo] Reserve adjacent LN_Last defs for the same phi use (PR #164577)

[Nikita Popov via llvm-commits]

nikic wrote:

To clarify, as it's not entirely clear from the PR description: Is this fixing a missed optimization issue (i.e. one of the predicates being lost on the phi edge)?

https://github.com/llvm/llvm-project/pull/164577