[llvm] [VPlan] Use SINGLE-SCALAR when printing single-scalar VPInstructions. (PR #141428)

[via llvm-commits]

================
@@ -965,7 +965,7 @@ void VPInstruction::dump() const {
 
 void VPInstruction::print(raw_ostream &O, const Twine &Indent,
                           VPSlotTracker &SlotTracker) const {
-  O << Indent << "EMIT ";
+  O << Indent << (isSingleScalar() ? "SINGLE-SCALAR " : "EMIT ");
----------------
ayalz wrote:

Would
```suggestion
  O << Indent << (isSingleScalar() ? "EMIT-SCALAR " : "EMIT ");
```
be better, keeping "EMIT" as common prefix for VPInstructions?

Cf. "CLONE" vs. "REPLICATE" of VPReplicateRecipe, where an underlying Inst is replicated-once=cloned, or more.

https://github.com/llvm/llvm-project/pull/141428