[SVE] Fix incorrect usage of getNumElements() in InstCombineCalls

Summary:
Remove incorrect usage of getNumElements() from visitCallInst(). The
number of elements was being used to construct a DemandedElts bitfield.
This operation does not make sense for scalable vectors. Cast to
FixedVectorType

Identified by test case Clang :: CodeGen/aarch64-sve-intrinsics/acle_sve_mla.c

Reviewers: rengolin, efriedma, sdesmalen, c-rhodes, david-arm

Reviewed By: david-arm

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D79524