[CodeGen] Fix warnings in getVectorElementCount()

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

Differential Revision: https://reviews.llvm.org/D81927

llvm/include/llvm/CodeGen/ValueTypes.h

@@ -299,7 +299,7 @@ namespace llvm {
       if (isSimple())
         return V.getVectorElementCount();
 
-      return {getExtendedVectorNumElements(), isExtendedScalableVector()};
+      return getExtendedVectorElementCount();
     }
 
     /// Given a vector type, return the minimum number of elements it contains.