[AArch64 NEON] Fix a pattern match failure with NEON_VDUP.

This failure caused by improper condition when lowering shuffle_vector
to scalar_to_vector. After this patch NEON_VDUP with v1i64 will not
be generated.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197966 91177308-0d34-0410-b5e6-96231b3b80d8
[AArch64 NEON] Fix a bug when lowering BUILD_VECTOR.

DAG.getVectorShuffle() doesn't always return a vector_shuffle node.
If mask is the exact sequence of it's operand(For example, operand_0
is v8i8, and  the mask is 0, 1, 2, 3, 4, 5, 6, 7), it will directly
return that operand. So a check is added here.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197967 91177308-0d34-0410-b5e6-96231b3b80d8