test/CodeGen: Check that the correct register is used in a store

This tightens up r208351 to ensure that a store is fed with the
correct value.

Thanks to Quentin Colombet for spotting this!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208368 91177308-0d34-0410-b5e6-96231b3b80d8