[AArch64] Fix a bug generating incorrect instruction when building small vector.

This bug is introduced by r211144. The element of operand may be
smaller than the element of result, but previous commit can
only handle the contrary condition. This commit is to handle this
scenario and generate optimized codes like ZIP1.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213830 91177308-0d34-0410-b5e6-96231b3b80d8