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

[Jerry Dang via llvm-commits]

kuroyukiasuna wrote:

@RKSimon This PR is becoming implementation for `SelectionDAG::computeKnownBits`  : )
I managed to get tests showing the optimization on AArch64 assembly. Please review when you have some time. Thanks!


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