[SVE] delete VectorType::getNumElements()

The previously agreed-upon deprecation period for
VectorType::getNumElements() has passed. This patch removes this method
and completes the refactor proposed in the RFC:
https://lists.llvm.org/pipermail/llvm-dev/2020-March/139811.html

Reviewed By: david-arm, rjmccall

Differential Revision: https://reviews.llvm.org/D95570