[llvm] [GlobalISel] Implement parity reasoning via G_CTPOP (PR #199078)

[Pawan Nirpal via llvm-commits]

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pawan-nirpal-031 wrote:

@arsenm  I don't see a point of introducing a pseudo opcode, unless there are some pre-isel Target specific combines that it can be used for. @jayfoad can you help us understand why it might be useful to have such a generic pseudo op. 

https://github.com/llvm/llvm-project/pull/199078