Fix another really nasty regression that Anshu pointed out. In cases where

Fix another really nasty regression that Anshu pointed out.  In cases where
dangling constant users were removed from a function, causing it to be dead,
we never removed the call graph edge from the external node to the function.

In most cases, this didn't cause a problem (by luck).  This should definitely
go into 1.3

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@15570 91177308-0d34-0410-b5e6-96231b3b80d8