[llvm] [RISCV] Combine chained `V[SZ]EXT_VL` ops (PR #211583)

[Craig Topper via llvm-commits]

topperc wrote:

> > That's weird that this is needed. Usually zexts of zexts are combined by the generic DAGCombiner whilst they're still ISD::ZERO_EXTENDs. What does the LLVM IR look like in the PR you've linked that ends up with the redundant extends?
> > Edit: oops nevermind, I see its coming from nodes during lowering.
> 
> I think this'll only happen on fixed vector version of ISD::ZERO_EXTEND though. But it also shows a sad reality that we might loose a bunch of generic DAGCombine patterns on fixed vectors because we have to convert them to *_VL ISD nodes early

Now that we have the VL optimizer, do we need the _VL nodes?

https://github.com/llvm/llvm-project/pull/211583