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

[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@239087 91177308-0d34-0410-b5e6-96231b3b80d8