[llvm] [VPlan] Encode single-scalar-ness of VPInstruction in a field (NFC). (PR #227761)

[Andrei Elovikov via llvm-commits]

eas wrote:

> Hm, so the idea would be to narrow recipes to single-scalars by simply narrowing its operands, and query the single-scalar-ness of recipes by checking if any of the operands are single-scalar?

I think the process/optimization and representation (`getSymbolicTypes` + VPlan printing) are orthogonal. Are you thinking about something like

```llvm
Live-ins:
  %bcast-ptr = ptr [unit, VF]
...
  %val = load i32 [unit, VF] %bcast-ptr
  store [VF] i32 %val, %other-bcast-ptr
```

transformed into

```llvm
Live-ins:
  %bcast-ptr = ptr [unit, VF]
...
  %val = load i32 [unit, VF] %bcast-ptr
  store [unit] i32 %val, %other-bcast-ptr
``` 

into

```llvm
Live-ins:
  %bcast-ptr = ptr [unit, VF]
...
  %val = load i32 [unit] %bcast-ptr
  store [unit] i32 %val, %other-bcast-ptr
``` 

and lastly

```llvm
Live-ins:
  %bcast-ptr = ptr [unit]
...
  %val = load i32 [unit] %bcast-ptr
  store [unit] i32 %val, %other-bcast-ptr
``` 

https://github.com/llvm/llvm-project/pull/227761