Merging r277755:

------------------------------------------------------------------------
r277755 | tnorthover | 2016-08-04 12:32:28 -0700 (Thu, 04 Aug 2016) | 5 lines

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.

------------------------------------------------------------------------

llvm-svn: 288845