llvm/test/CodeGen/AArch64/neon-scalar-reduce-pairwise.ll: Use -mtriple here, or aach64-pecoff might be misassumed on win32 hosts.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191275 91177308-0d34-0410-b5e6-96231b3b80d8