Totally gut mergeInGraph. There is absolutely no reason to be merging

global roots in from callees to callers.  The BU graphs do not have accurate
globals information and all of the clients know it.  Instead, just make sure
the GG is up-to-date, and they will be perfectly satiated.

This speeds up the BU pass on 176.gcc from 5.5s to 1.5s, and Loc+BU+TD
from 7s to 2.7s.

llvm-svn: 20786