[ARM64]Fix a bug when lowering shuffle vector to an EXT instruction.
E.g. Mask like <-1, -1, 1, ...> will generate incorrect EXT index.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207485 91177308-0d34-0410-b5e6-96231b3b80d8