Implement a new mergeInGraph method, which basically factors code out of

Implement a new mergeInGraph method, which basically factors code out of
the BU class.

This will be used by the IPModRef class to do stuff, eventually perhaps the
TD pass will use it also.

Speaking of the TD pass, this also eliminates the self recursive case, which
was broken, and couldn't occur anyway.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@4599 91177308-0d34-0410-b5e6-96231b3b80d8