[InstCombine] relax constraints on mask-of-add

There are 2 changes:
1. Remove the unnecessary one-use check.
2. Remove the unnecessary power-of-2 check.

https://rise4fun.com/Alive/V6fP

  Name: add with low mask
  Pre: (C1 & (-1 u>> countLeadingZeros(C2))) == 0
  %a = add i8 %x, C1
  %r = and i8 %a, C2
  =>
  %r = and i8 %x, C2