[InstCombine] Narrow llvm.abs through trunc. (#199643)
Update EvaluateInDifferentType / canEvaluateTruncated to narrow abs
intrinsics when the operand has at least OrigBitWidth - BitWidth + 1
sign bits. The transform always emits the narrow abs with
IsIntMinPoison=false, as the narrowed value may be INT_MIN in the narrow
type, while not in the original width.
Alive2 Proof with weaker precondition (top and truncated sign bits must
match): https://alive2.llvm.org/ce/z/AMQRmi
End-to-end C pixel math example: https://clang.godbolt.org/z/Ma8bsTGTY
PR: https://github.com/llvm/llvm-project/pull/199643
