[ValueTracking] Improve the coverage of isKnownToBeAPowerOfTwo for vscale

[Allen]

Summary

this PR tries to match the following pattern, seperate from D156881

%vscale = call i64 @llvm.vscale.i64()
%shift = shl nuw nsw i64 %vscale, 11

Now, we only check the shl recursively when the OrZero is true.

[nikic]

llvm/lib/Analysis/ValueTracking.cpp

2022

This isn't correct. A shift left of vscale can be zero if all bits get shifted out. There is no special interaction between vscale and shl.

[goldstein.w.n]

I'm going to revert this. Also have patch in the works that should cover this.

llvm/lib/Analysis/ValueTracking.cpp

bool isKnownToBeAPowerOfTwo(const Value *V, bool OrZero, unsigned Depth,
                             const SimplifyQuery &Q) {
   assert(Depth <= MaxAnalysisRecursionDepth && "Limit Search Depth");
 
   // Attempt to match against constants.
   if (OrZero && match(V, m_Power2OrZero()))
       return true;
   if (match(V, m_Power2()))
       return true;
-  if (Q.CxtI && match(V, m_VScale())) {
+  if (Q.CxtI &&
+      (match(V, m_VScale()) || match(V, m_Shl(m_VScale(), m_Value())))) {
     const Function *F = Q.CxtI->getFunction();
     // The vscale_range indicates vscale is a power-of-two.
     return F->hasFnAttribute(Attribute::VScaleRange);
   }
 
   // 1 << X is clearly a power of two if the one is not shifted off the end.  If
   // it is shifted off the end then the result is undefined.
   if (match(V, m_Shl(m_One(), m_Value())))