[AArch64]Fix the pattern match failure for v1i8/v1i16/v1i32 types.
Currently we have such types as legal vector types. The DAG combiner may generate some DAG nodes having such types but we don't have patterns to match them.
E.g. a load i32 and a bitcast i32 to v1i32 will be combined into a load v1i32:
     bitcast (load i32) to v1i32 -> load v1i32.
So this patch fixes such problems for load/dup instructions.
If v1i8/v1i16/v1i32 are not legal any more, the code in this patch can be deleted. So I also add some FIXME.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197361 91177308-0d34-0410-b5e6-96231b3b80d8