[llvm] [DAG]SimplifyDemandedVectorElts-add ISD::AVGCEILS/AVGCEILU/AVGFLOORS/AVGFLOORU nodes (PR #86284)

[Simon Pilgrim via llvm-commits]

================
@@ -879,6 +879,58 @@ define <8 x i16> @uhadd_fixedwidth_v4i32(<8 x i16> %a0, <8 x i16> %a1)  {
   ret <8 x i16> %res
 }
 
+define <8 x i16> @shadd_demandedelts(<8 x i16> %a0, <8 x i16> %a1) {
+; CHECK-LABEL: shadd_demandedelts:
+; CHECK:       // %bb.0:
+; CHECK-NEXT:    dup v0.8h, v0.h[0]
+; CHECK-NEXT:    shadd v0.8h, v0.8h, v1.8h
+; CHECK-NEXT:    dup v0.8h, v0.h[0]
+; CHECK-NEXT:    ret
----------------
RKSimon wrote:

These don't appear to be working? I'd expect:
```
; CHECK:       // %bb.0:
; CHECK-NEXT:    shadd v0.8h, v0.8h, v1.8h
; CHECK-NEXT:    dup v0.8h, v0.h[0]
; CHECK-NEXT:    ret
```

https://github.com/llvm/llvm-project/pull/86284