[llvm] [DAG] visitCTPOP - if only the upper half of the ctpop operand is zero then see if its profitable to only count the lower half. (PR #80473)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

I mean with 3a758076f54d521d20e32856a62c7d24803ee9e8 we can now search all integer ctpop ops for "shifted active bits" cases that allow us to identify when we can make use of more of fast implementations listed in #79823 

https://github.com/llvm/llvm-project/pull/80473