[PPC64LE] Update test for vec_sum2s interface

Commit r210384 prematurely included changes to the little-endian
implementation of the vec_sum2s interface.  This patch modifies
test/CodeGen/builtins-ppc-altivec.c to test those changes.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@210389 91177308-0d34-0410-b5e6-96231b3b80d8