This commit allows vectorized loops to be unrolled by a factor of 2 for AArch64.
A new test case is also added for ARM64.

Patched by Z.Zheng



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206563 91177308-0d34-0410-b5e6-96231b3b80d8