[SVE] Mark VectorType::getNumElements() deprecated

getNumElements() is being removed from base VectorType in
order to eliminate the class of bugs in which a scalable vector
is accidentally treated like a fixed length vector. Clients of
this function should either call getElementCount(), and handle
the case where getElementCount().isScalable() is true, or they can
cast to FixedVectorType and call getNumElements() if they are
sure that the vector has fixed width.

Deprecated VectorType functions will be removed after the LLVM
12 branch.

See: http://lists.llvm.org/pipermail/llvm-dev/2020-March/139811.html

Reviewed By: fpetrogalli

Differential Revision: https://reviews.llvm.org/D78127