Implement the newly added AArch64 ACLE functions for ld1/st1 with 2/3/4 vectors.
The functions are like: vst1_s8_x2 ...


git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@194991 91177308-0d34-0410-b5e6-96231b3b80d8