[LoopVectorize] Up the maximum unroll factor to 4 for AArch64

Only for Cortex-A57 and Cyclone for now, where it has shown wins.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216141 91177308-0d34-0410-b5e6-96231b3b80d8