[ARM] Prevent PerformVCVTCombine from combining a vmul/vcvt with 8 lanes

This would result in a crash since the vcvt used does not support v8i32 types.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224332 91177308-0d34-0410-b5e6-96231b3b80d8