[SVE] Fix bad FixedVectorType cast in simplifyDivRem

Summary:
simplifyDivRem attempts to walk a VectorType elementwise. Ensure that it
only does so for FixedVectorType

Reviewers: efriedma, spatel, lebedev.ri, david-arm, kmclaughlin

Reviewed By: spatel, david-arm

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D81856