[DAG] Do not call computeKnownBits from isKnownToBeAPowerOfTwo

[foad]

Summary

The only way known bits could help identify a known power of two is if
it knows exactly which power of two it is, i.e. if it is a known
constant. But in that case the value should have been simplified to a
constant already. So save some compile time by not calling
computeKnownBits.

[RKSimon]

LGTM - IIRC the plan was to add DemandedElts support which would have made better use of this but it has never ended up being necessary

llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

bool SelectionDAG::isKnownToBeAPowerOfTwo(SDValue Val) const {
   // vscale(power-of-two) is a power-of-two for some targets
   if (Val.getOpcode() == ISD::VSCALE &&
       getTargetLoweringInfo().isVScaleKnownToBeAPowerOfTwo() &&
       isKnownToBeAPowerOfTwo(Val.getOperand(0)))
     return true;
 
   // More could be done here, though the above checks are enough
   // to handle some common cases.
-
-  // Fall back to computeKnownBits to catch other known cases.
-  KnownBits Known = computeKnownBits(Val);
-  return (Known.countMaxPopulation() == 1) && (Known.countMinPopulation() == 1);
+  return false;
 }
 
 unsigned SelectionDAG::ComputeNumSignBits(SDValue Op, unsigned Depth) const {
   EVT VT = Op.getValueType();
 
   // Since the number of lanes in a scalable vector is unknown at compile time,
   // we track one bit which is implicitly broadcast to all lanes.  This means
   // that all lanes in a scalable vector are considered demanded.