[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