[llvm] [InstCombine][RISCV] Convert VPIntrinsics with splat operands to splats (PR #65706)

[Luke Lau via llvm-commits]

================
@@ -729,6 +730,96 @@ bool VectorCombine::foldBitcastShuf(Instruction &I) {
   return true;
 }
 
+/// VP Intrinsics whose vector operands are both splat values may be simplified
+/// into the scalar version of the operation and the result is splatted. This
+/// can lead to scalarization down the line.
+bool VectorCombine::scalarizeVPIntrinsic(VPIntrinsic &VPI) {
+  Value *Op0 = VPI.getArgOperand(0);
+  Value *Op1 = VPI.getArgOperand(1);
+
+  if (!getSplatValue(Op0) || !getSplatValue(Op1))
+    return false;
+
+  // For the binary VP intrinsics supported here, the result on disabled lanes
+  // is a poison value. For now, only do this simplification if all lanes
+  // are active.
+  // TODO: Relax the condition that all lanes are active by using insertelement
+  // on inactive lanes.
+  auto IsAllTrueMask = [](Value *MaskVal) {
+    if (Value *SplattedVal = getSplatValue(MaskVal))
+      if (auto *ConstValue = dyn_cast<Constant>(SplattedVal))
+        return ConstValue->isAllOnesValue();
+    return false;
+  };
+  if (!IsAllTrueMask(VPI.getArgOperand(2)))
+    return false;
----------------
lukel97 wrote:

Nit: I think `Constant::isAllOnesValue()` already checks if it's a constant splat vector, can we simplify this to something like

```suggestion
  if (!isa<Constant>(Mask) || !cast<Constant>(Mask)->isAllOnesValue())
    return false;
```

https://github.com/llvm/llvm-project/pull/65706