[InstCombine] allow vectors for masked-add -> xor fold

https://rise4fun.com/Alive/I4Ge

  Name: add with pow2 mask
  Pre: isPowerOf2(C2) && (C1 & C2) != 0 && (C1 & (C2-1)) == 0
  %a = add i8 %x, C1
  %r = and i8 %a, C2
  =>
  %n = and i8 %x, C2
  %r = xor i8 %n, C2