[llvm] [DAG] Handle truncated splat in isBoolConstant (PR #145473)

[David Green via llvm-commits]

davemgreen wrote:

Hi - I'm Ok either way, up to you what you think is best. If you want to go with #145724 then I can give this a rebase. If you prefer not to support the negations in PreprocessISelDAG then we can go with this method. Either way thanks for the assistance.

https://github.com/llvm/llvm-project/pull/145473