[LCG] When removing a dead function and clearing out the data

[LCG] When removing a dead function and clearing out the data
structures, actually null out the graph pointers as well. We won't ever
update these, and we certainly shouldn't be calling any methods on them,
so it seems good to defensively nuke them.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@310164 91177308-0d34-0410-b5e6-96231b3b80d8