[SVE] Remove invalid calls to VectorType::getNumElements from BasicTTIImpl

Summary:
Most of these operations are reasonable for scalable vectors. Due to
this, we have decided not to change the interface to specifically take
FixedVectorType despite the fact that the current implementations make
fixed width assumptions. Instead, we cast to FixedVectorType and assert
in the body. If a developer makes some change in the future that causes
one of these asserts to fire, they should either change their code or
make the function they are trying to call handle scalable vectors.

Reviewers: efriedma, samparker, RKSimon, craig.topper, sdesmalen, c-rhodes

Reviewed By: efriedma

Subscribers: tschuett, rkruppe, psnobl, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D81495