[llvm] [LangRef] Require that vscale be a power of two (PR #145098)

[Paul Walker via llvm-commits]

================
@@ -4660,7 +4660,6 @@ bool SelectionDAG::isKnownToBeAPowerOfTwo(SDValue Val, unsigned Depth) const {
 
   // vscale(power-of-two) is a power-of-two for some targets
   if (Val.getOpcode() == ISD::VSCALE &&
-      getTargetLoweringInfo().isVScaleKnownToBeAPowerOfTwo() &&
       isKnownToBeAPowerOfTwo(Val.getOperand(0), Depth + 1))
     return true;
----------------
paulwalker-arm wrote:

```
if (Val.getOpcode() == ISD::VSCALE)
  return isKnownToBeAPowerOfTwo(Val.getOperand(0), Depth + 1);
```

https://github.com/llvm/llvm-project/pull/145098