[CodeGen] Fix warnings in getVectorElementCount()

[david-arm]

Summary

In EVT::getVectorElementCount() when the type is not simple we
should return getExtendedVectorElementCount() from the function
instead of constructing the ElementCount object manually.

I discovered this warning in an existing test:

test/CodeGen/AArch64/sve-intrinsics-loads.ll

llvm/include/llvm/CodeGen/ValueTypes.h

#endif
     }
 
     // Given a (possibly scalable) vector type, return the ElementCount
     ElementCount getVectorElementCount() const {
       assert((isVector()) && "Invalid vector type!");
       if (isSimple())
         return V.getVectorElementCount();
 
-      return {getExtendedVectorNumElements(), isExtendedScalableVector()};
+      return getExtendedVectorElementCount();
     }
 
     /// Given a vector type, return the minimum number of elements it contains.
     unsigned getVectorMinNumElements() const {
       return getVectorElementCount().Min;
     }
 
     /// Return the size of the specified value type in bits.