[InstCombine] Add folds for `(icmp spred (ssub.sat X, Y), 0)` -> `X spred Y`

Alive2 links:
    eq: https://alive2.llvm.org/ce/z/Fv3mvc
    ne: https://alive2.llvm.org/ce/z/AEuEXU
    sle: https://alive2.llvm.org/ce/z/mfKGUS
    sge: https://alive2.llvm.org/ce/z/tX3_M4
    sgt: https://alive2.llvm.org/ce/z/x7VgnZ
    slt: https://alive2.llvm.org/ce/z/rQN4TM

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D149521