AArch64: Follow-up to r212073

In r212073 I missed a call of `use_begin()` that assumed the wrong
semantics.  It's not clear to me at all what this code does without the
fix, so I'm not sure how to write a testcase.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212075 91177308-0d34-0410-b5e6-96231b3b80d8