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

[Craig Topper via llvm-commits]

topperc wrote:

> The difficulty I ended up running into is that KnownBits doesn't store enough information about the potential range of the result. I.e. an i64 sub of two zero extended i8 integers will be between 0x00000000000000FF and 0xFFFFFFFFFFFFFE02. So the range can fit into i16, but from KnownBits perspective this is just 0x??????????????. So we can't actually tell what the minimum bit width needed is.
> 

Does ComputeNumSignBits know?

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