[InstCombine] Fold abs of known sign operand when source is sub

abs(x-y) --> x-y where x >= y, done on D122013
abs(x-y) --> y-x where x <= y

proofs: https://alive2.llvm.org/ce/z/KkeEsd

Reviewed By: goldstein.w.n, nikic
Differential Revision: https://reviews.llvm.org/D156499