[llvm] [InstCombine] Narrow the ctpop when upper bits are known zero (PR #196537)

[Rutansh Suthar via llvm-commits]

RutanshS wrote:

Addressed review comments. Removed magic numbers, now narrows to bit_ceil(ActiveBits) which handles non-pow2 types like i65 correctly. Added i65 regression test. I skipped a minimum width to keep it target independent.

https://github.com/llvm/llvm-project/pull/196537