[GlobalMerge] Take into account minsize on Global users' parents.

Now that we can look at users, we can trivially do this: when we would
have otherwise disabled GlobalMerge (currently -O<3), we can just run
it for minsize functions, as it's usually a codesize win.

Differential Revision: http://reviews.llvm.org/D10054

llvm-svn: 239087