[AArch64] Fix incorrect use of MVT::getVectorNumElements in AArch64TTIImpl::getVectorInstrCost

If we are inserting into or extracting from a scalable vector we do
not know the number of elements at runtime, so we can only let the
index wrap for fixed-length vectors.

Tests added here:

  Analysis/CostModel/AArch64/sve-insert-extract.ll

Differential Revision: https://reviews.llvm.org/D117099