[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