Inplement aarch64 neon instructions in AdvSIMD(shift). About 24 shift instructions:

      sshr,ushr,ssra,usra,srshr,urshr,srsra,ursra,sri,shl,sli,sqshlu,sqshl,uqshl,shrn,sqrshrun,sqshrn,uqshr,sqrshrn,uqrshrn,sshll,ushll
 and 4 convert instructions:
      scvtf,ucvtf,fcvtzs,fcvtzu

llvm-svn: 189925