[ConstantFold] Handle undef/poison when constant folding smul_fix/smul_fix_sat

Do constant folding according to
  posion * C -> poison
  C * poison -> poison
  undef * C -> 0
  C * undef -> 0
for smul_fix and smul_fix_sat intrinsics (for any scale).

Reviewed By: nikic, aqjune, nagisa

Differential Revision: https://reviews.llvm.org/D98410