Global DCE performance improvement

Change the original algorithm so that it scales better when meeting
very large bitcode where every instruction does not implies a global.

The target query is "how to you get all the globals referenced by
another global"?

Before this patch, it was doing this by walking the body (or the
initializer) and collecting the references. What this patch is doing,
it precomputing the answer to this query for the whole module by
walking the use-list of every global instead.

Patch by: Serge Guelton <serge.guelton@telecom-bretagne.eu>

Differential Revision: https://reviews.llvm.org/D28549

llvm-svn: 293328