AArch64: only try to get operand of a known node.

A bug in r216725 meant we tried to discover the type of a SETCC before
confirming the node actually was a SETCC.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216734 91177308-0d34-0410-b5e6-96231b3b80d8