[InstCombine] fold 'not' of ctpop in parity pattern

As discussed in https://llvm.org/PR50096 , we could
convert the 'not' into a 'sub' and see the same
fold. That's because we already have another demanded
bits optimization for 'sub'.

We could add a related transform for
odd-number-of-type-bits, but that seems unlikely
to be practical.

https://alive2.llvm.org/ce/z/TWJZXr