AArch64: don't assume all i128s are BUILD_PAIRs

It leads to a crash when they're not. I'm *sure* I've made this mistake before,
at least once.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@277755 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed