Added functions cross-reference test.
Originally this similar was initiated by Björn Steinbrink here:
http://reviews.llvm.org/D3437

Bug itself has been fixed by principal changes in MergeFunctions. Though
special checks for functions merging are still actual. And the test has
been accepted with slight modifications.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210486 91177308-0d34-0410-b5e6-96231b3b80d8