[llvm] [DAG] ISD::matchUnaryPredicate / matchUnaryFpPredicate / matchBinaryPredicate - add DemandedElts variant (PR #183013)

[Matt Arsenault via llvm-commits]

================
@@ -361,8 +361,14 @@ bool ISD::matchUnaryPredicateImpl(SDValue Op,
       ISD::SPLAT_VECTOR != Op.getOpcode())
     return false;
 
+  if (ISD::SPLAT_VECTOR == Op.getOpcode() && DemandedElts == 0)
----------------
arsenm wrote:

```suggestion
  if (ISD::SPLAT_VECTOR == Op.getOpcode() && !DemandedElts)
```

https://github.com/llvm/llvm-project/pull/183013