[llvm] [SelectionDAG] Add TRUNCATE_SSAT_S/U and TRUNCATE_USAT_U to canCreateUndefOrPoison and computeKnownBits (#152143) (PR #168809)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon requested changes to this pull request.

@kuroyukiasuna Apologies - this dropped off my radar (i.e. first page of review list) - please can you merge against trunk latest and I'll get back to this

https://github.com/llvm/llvm-project/pull/168809