[llvm] [RISCV] Teach combineBinOpOfZExt to narrow based on known bits (PR #86680)

[Philip Reames via llvm-commits]

preames wrote:

I knew this was somewhat WIP when I posted it, but I hadn't realized I had several nasty bugs which invalidated all the test changes.  Going to start this one from scratch with a new review once I actually have something which works.  

https://github.com/llvm/llvm-project/pull/86680