[llvm] [InstCombine] Remove over-generalization from computeKnownBitsFromCmp() (PR #72637)

[via llvm-commits]

goldsteinn wrote:

Some of the simplifications here make sense independent of general -> constant simplification. But for the general -> constant simplification, imo it makes more sense to add coverage as opposed to just dropping the code.
Particularly a few cases like `X == Y`, think its simple enough to support non-constant `Y` and can imagine practical value from that type of analysis.

https://github.com/llvm/llvm-project/pull/72637