[InstCombine] remove unnecessary one-use check from add-xor transform

Pre-conditions seem to be optimal, but we don't need a use check
because we are only replacing an add with a sub.

https://rise4fun.com/Alive/hzN

  Pre: (~C1 | C2 == -1) && isPowerOf2(C2+1)
  %m = and i8 %x, C1
  %f = xor i8 %m, C2
  %r = add i8 %f, C3
  =>
  %r = sub i8 C2 + C3, %m