[AArch64] Fix a use of uninitialized memory introduced in r203125,
and caught by the MSan bootstrap build bot. This should hopefully get
the bot green at long last.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203441 91177308-0d34-0410-b5e6-96231b3b80d8