Adjust test/CodeGenCXX/pr12251.cpp

InstCombine just got a bit smarter about checking known bits of returned
values, and because this test runs the optimizer, it requires an update. We
should really rewrite this test to directly check the IR output from CodeGen.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@217347 91177308-0d34-0410-b5e6-96231b3b80d8