[SVE] Fix invalid usage of getNumElements() in InstCombineMulDivRem

Summary:
getLogBase2 tries to iterate over the number of vector elements. Since
the number of elements of a scalable vector is unknown at compile time,
we must return null if the input type is scalable.

Identified by test LLVM.Transforms/InstCombine::nsw.ll

Reviewers: efriedma, fpetrogalli, kmclaughlin, spatel

Reviewed By: efriedma, fpetrogalli

Subscribers: tschuett, hiraditya, rkruppe, psnobl, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D79197