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

[Philip Reames via llvm-commits]

preames wrote:

I was going to drop a comment on https://github.com/llvm/llvm-project/pull/86680 warning that whichever patch landed first would need rebased carefully as the required zero bits are different for mul than for add/sub, but Luke beat me to landing between when I decided to post this and actually doing so.  As such, I will need to rebase this to account for the newly added mul case.  :)

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