[llvm] [RISCV] Add DAG combine for forming VAADDU_VL from VP intrinsics. (PR #124848)

[Craig Topper via llvm-commits]

topperc wrote:

> > > LGTM
> > > Could we also do this after legalization and try and match it on `TRUNCATE_VECTOR_VL`? That way we would also match fixed length vectors.
> > 
> > 
> > Don't we have a generic DAGCombine for fixed vectors using non-VP operations?
> 
> Yeah but this would be for fixed length VP operations. Just putting the idea out there

I think moving it after legalization is a little more complex. We'll have to handle VP_TRUNCATE i32->i8 being split into 2 TRUNCATE_VECTOR_VLs and we'll have to look for combinations of VWADDU_VL and VWADDU_W_VL.

https://github.com/llvm/llvm-project/pull/124848