[CodeGen] Fix ComputeNumSignBits for scalable vectors

[david-arm]

Summary

When trying to calculate the number of sign bits for scalable vectors
we should just bail out for now and pretend we know nothing.

[david-arm]

ping

[RKSimon]

LGTM

llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

bool SelectionDAG::isKnownToBeAPowerOfTwo(SDValue Val) const {
 
   // Fall back to computeKnownBits to catch other known cases.
   KnownBits Known = computeKnownBits(Val);
   return (Known.countMaxPopulation() == 1) && (Known.countMinPopulation() == 1);
 }
 
 unsigned SelectionDAG::ComputeNumSignBits(SDValue Op, unsigned Depth) const {
   EVT VT = Op.getValueType();
-  APInt DemandedElts = VT.isVector()
-                           ? APInt::getAllOnesValue(VT.getVectorNumElements())
-                           : APInt(1, 1);
+  APInt DemandedElts;
+  if (!VT.isScalableVector()) {
+    DemandedElts = VT.isVector()
+  }
   return ComputeNumSignBits(Op, DemandedElts, Depth);
 }
 
 unsigned SelectionDAG::ComputeNumSignBits(SDValue Op, const APInt &DemandedElts,
                                           unsigned Depth) const {
   EVT VT = Op.getValueType();
   assert((VT.isInteger() || VT.isFloatingPoint()) && "Invalid VT!");
   unsigned VTBits = VT.getScalarSizeInBits();
   unsigned NumElts = DemandedElts.getBitWidth();
   unsigned Tmp, Tmp2;
   unsigned FirstAnswer = 1;
 
   if (auto *C = dyn_cast<ConstantSDNode>(Op)) {
     const APInt &Val = C->getAPIntValue();
     return Val.getNumSignBits();
   }
 
   if (Depth >= MaxRecursionDepth)
     return 1;  // Limit search depth.
 
-  if (!DemandedElts)
+  if (!DemandedElts || VT.isScalableVector())
     return 1;  // No demanded elts, better to assume we don't know anything.
 
   unsigned Opcode = Op.getOpcode();
   switch (Opcode) {
   default: break;
   case ISD::AssertSext:
     Tmp = cast<VTSDNode>(Op.getOperand(1))->getVT().getSizeInBits();
     return VTBits-Tmp+1;