Don't merge two constants if we care about the address of both.

Don't merge two constants if we care about the address of both.

This fixes the original testcase in PR8927. It also causes a clang
binary built with a patched clang to increase in size by 0.21%.

We can probably get some of the size back by writing a pass that
detects that a global never has its pointer compared and adds
unnamed_addr to it (maybe extend global opt). It is also possible that
there are some other cases clang could add unnamed_addr to.

I will investigate extending globalopt next.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@123584 91177308-0d34-0410-b5e6-96231b3b80d8