[CodeGen] Fix warnings in isPow2VectorType and getPow2VectorType

[david-arm]

Summary

We should either call getVectorMinNumElements() or
getVectorElementCount().

llvm/include/llvm/CodeGen/ValueTypes.h

EVT getHalfNumVectorElementsVT(LLVMContext &Context) const {
       EVT EltVT = getVectorElementType();
       auto EltCnt = getVectorElementCount();
       assert(!(EltCnt.Min & 1) && "Splitting vector, but not in half!");
       return EVT::getVectorVT(Context, EltVT, EltCnt / 2);
     }
 
     /// Returns true if the given vector is a power of 2.
     bool isPow2VectorType() const {
-      unsigned NElts = getVectorNumElements();
+      unsigned NElts = getVectorMinNumElements();
       return !(NElts & (NElts - 1));
     }
 
     /// Widens the length of the given vector EVT up to the nearest power of 2
     /// and returns that type.
     EVT getPow2VectorType(LLVMContext &Context) const {
       if (!isPow2VectorType()) {
-        unsigned NElts = getVectorNumElements();
-        unsigned Pow2NElts = 1 <<  Log2_32_Ceil(NElts);
-        return EVT::getVectorVT(Context, getVectorElementType(), Pow2NElts,
-                                isScalableVector());
+        ElementCount NElts = getVectorElementCount();
+        NElts.Min = 1 << Log2_32_Ceil(NElts.Min);
+        return EVT::getVectorVT(Context, getVectorElementType(), NElts);
       }
       else {
         return *this;
       }
     }
 
     /// This function returns value type as a string, e.g. "i32".
     std::string getEVTString() const;