[SVE] Remove invalid usage of VectorType::getNumElements in Function

Summary:
Removes usage of VectorType::getNumElements identified by test located
at CodeGen/aarch64-sve-intrinsics/acle_sve_dot.c. This code explicitly
converts a potentially fixed length vector to scalable vector by
constructing the ElementCount = {getNumElements(), true}

Reviewers: rengolin, efriedma, kmclaughlin, c-rhodes, sdesmalen

Reviewed By: efriedma

Subscribers: tschuett, kristof.beyls, hiraditya, rkruppe, psnobl, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78967