[llvm] [CodeGen] Optimize ctpop expansion using computeKnownBits (PR #221671)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon commented:

PTAL at LowerCTPOP in X86ISelLowering.cpp - which uses a shifted active mask for this kind of thing (some of which can probably be moved to expandCTPOP).

https://github.com/llvm/llvm-project/pull/221671